Bismillah al-Rahman al-Rahim
“It is He who created the heavens and the earth in true (proportions): the day He saith, 'Be,' behold! it is. His word is the truth. His will be the dominion the day the trumpet will be blown. He knoweth the unseen as well as that which is open. For He is the Wise, well acquainted (with all things).” (Qur'an 6:73)
A trumpet is traditionally an instrument of announcement - it is used to announce an event, or the commencement of something important to all the people. For example, it may herald the return of a king to his subjects - and a time of accounting follows this arrival as the king asks all those who were his representatives and who were responsible for his kingdom to account for the current state of the kingdom. So it can also indicate an announcement of the commencement of a time of accounting and judgement - a time when the dominion, rulership, and judgement passes directly back into the hands of the king and the entire kingdom is called to account for what has transpired within it.
The blowing of the trumpet in this verse signifies the calling of everything that is in the heavens and earth to an accounting. It signifies a transformation of all existence as described in surah 69:13-17.
“And when the trumpet is blown with a single blast, And the earth and the mountains are borne away and dissolved like powder.... On that day you shall be exposed to view - no secret of yours shall remain hidden.” (Qur'an 69:13-17)
When the Prophet was asked about this trumpet he said, “It is a horn of light upon which Israfil blows.”
And the blowing of the trumpet is a twofold blowing, not a single blowing. The Qur'an says, “The trumpet will be sounded, when all that is in the heavens and the earth will swoon away, except such as it will please God to exempt. Then will a second one be sounded, when, behold, they will be standing and looking on!” (Qur’an 39:68)
So, what is a possible meaning of the two blowings of the trumpet?
God created the heavens and earth with Truth. He creates with the creative command: “Be!” And whatever he commands enters into being. This creative "Be" encompasses all times and places. Since God is beyond time and place (being the creator of time and place), His command enters from beyond these limited realms and manifests itself in this world of time, place, and form in the best, most appropriate manner. His creative command takes a shape, a form when it enters this universe we live in. It can enter into this world in multiple places and times as necessary to create what needs to be created - activating whatever sequences of cause and effect and whatever intersecting destinies are required to create what needs to be created and cause what needs to be caused. So underlying all the forms of this world is the Truth and Power of commands that are veiled from us by the forms and limits of existence and by the boundaries of our perceptions and the fact that all our instruments of perception operate within the limits of this universe. We are limited in our ability to perceive form, place, and time and we are limited from perceiving what is beyond form, place, and time.
The process of thought, memory, and intellect nevertheless allows us to push our understanding somewhat beyond the limits of form and matter. By use of our intellect we are able to penetrate to some extent beyond the forms of the material world. But our understanding of these matters is shadowy, vague, theoretical, speculative, filled with mystery. It is not a clear direct perception but a fuzzy, shadowy perception.
Light illuminates - it reveals what was concealed, lifts the mystery of things that were in shadow by illuminating and revealing what was not clear. Israfil's trumpet is a trumpet of light - an illuminating trumpet that reveals with clarity what was only dimly perceived or understood, what was shrouded in the darkness of matter and the darkness of our limited understanding. It illuminates and removes the veils from our intellect making possible a piercing and deep understanding.
God has created everything in existence with a hidden Truth and Power. We have inklings of this through our material science. The incredible power concealed within the most minute particles - within tiny units of matter. Likewise, we are told that humans have been created with concealed and concentrated power and with truth. Our physical form is one of weakness and animality, but this is like a veil over the potentialities within us, just as the minuteness of the atom is a veil over the power concealed within it.
When God creates the forms of this world (the heavens and the earth) and gives them their shape and characteristics, their time and place, He also gives them their preparedness to be resurrected. For example, when coal is heated it has within it a readiness to be set aflame. But the fire within it only comes into view, only engulfs the coal when you blow upon it. The blowing of the trumpet is like a blowing that reveals the hidden nature and truth of existence.
The trumpet of light then is a clarifying trumpet which reveals the true nature of existence - and human perception is made keen by it, and human consciousness of the spiritual world is made clear. So the human souls have everything made piercingly clear to them, and whoever “...has done an atom's weight of good shall see it. And whoever who has done an atom's weight of evil shall see it....” (Qur'an 99:7-8) and each soul knows its true rank.
The first blowing of the trumpet then transforms everything so that hidden potentials and truths are made manifest - everything transcends its material existence (its external physical form) - the veil of the physical world is removed with the first blowing.
The second blowing is the resurrection. The verse says, “....and lo, they are standing and are looking on! And the earth shines forth with the Light of its Lord.” (39:68-69) Some of those risen are pleased with their condition while others say “Woe unto us” - each speaking according to their state of being. When all matters are piercingly illuminated, some are pleased to stand in the light while others become ashamed when the light exposes their true spiritual condition - their inner selves. Rumi puts it like this:
“This world is like mid-winter. Why are inanimate objects called solid. It is because they are frozen. These rocks mountains and other coverings that garb this world are all frozen. If the world is not mid-winter, why is it frozen?.... This world is like the season of mid-winter when everything is frozen and solidified, What sort of mid-winter? A mental mid-winter, not a tangible one. When that 'divine' breeze comes, the mountains of this world will begin to melt and turn to water. Just as the heat of midsummer causes all frozen things to melt, so on the day of resurrection, when that breeze comes, all things will melt.” (Rumi)
As the verses say, the heavens and earth will be rent asunder, the mountains will lose all form - everything will swoon away - the frozen, material forms will subside and everything will reveal its hidden form. (The earth will “shine with the Light of it's Lord”) - everything will be placed in its proper position with respect to it's origin and source - God. 
So God created everything with Truth. Everything is created with His creative word - “Be!” - and His Word is the Truth and carries the truth into creation. And when the trumpet is blown, everything will have it's truth revealed and the nature of all things will be uncovered and lit up by the trumpet of light. For God everything is always manifest - both the unseen and the seen - and He will, with the blast of the trumpet uncover and illuminate much of that which was unseen, revealing it to His creation.
Abu Basir has related that he said to Abu Abdallah (Imam Jafar-al-Sadiq) - upon whom be peace, “Tell me about God, the Mighty and Majestic Will believers see Him on the Day of Resurrection?”
He answered, “Yes, and they have already seen Him before the Day of Resurrection.”
Abu Basir asked, “When?”
The Imam answered, “When He said to them in pre-eternity, 'Am I not your Lord?' They said: 'Yea, verily' (Qur'an 7, I72).” Then he was quiet for a time. Then he said, “Truly the believers see him in this world before the Day of Resurrection. Doest thou not see Him now?”
Abu Basir then said to him, “That I might be made thy sacrifice, Shall I relate this (to others) from thee?”
He answered, “No, for if thou relatest it, a denier ignorant of the meaning of what thou sayest will deny it. Then he will suppose that it is comparison and unbelief (kufr). But seeing with the heart (al-ru'yah b-il-qalb) is not like seeing with the eyes (al- ru'yah bi-l-ayn). High be God exalted above what the comparers (mushabbihun) and heretics (mulhidun) describe!”
(Imam Jafar al-Sadiq (a.s.) on perceiving with the heart - from “A Shi'ite Anthology”)
His names are an expression (tabir), His acts (afal) are (a way) to make (Him) understood (tafhim), and His Essence is Reality (haqiqah). His inmost center (kunh) separates (tafriq) Him from creation, and His otherness (ghuyur) limits (tahdid) what is other than He. Therefore ignorant of God is he who asks for Him to be described! Transgressing against Him is he who seeks to encompass Him! Mistaken is he who imagines to have fathomed Him!
Whoso says 'how?' has compared Him (to something). Whoso says 'why?' has professed for Him a cause (talil). Whoso says 'when?' has determined Him in time (tawqit). Whoso says 'in what?' has enclosed Him (tadmin). Whoso says 'to what?' has professed for Him a limit (tanhiyah). Whoso says 'until what?' has given Him an end (taghiyah). Whoso gives Him an end has associated an end with Him. Whoso associates an end with Him has divided Him. Whoso divides Him has described Him. Whoso describes Him has deviated from the straight path (ilhad) concerning Him.
God does not change with the changes undergone by creation, just as He does not become limited by delimiting (tahdid) that which is limited (al-mahdud). He is One (ahad), not according to the explanation offered by number (tawil adad); Outward, not according to the explanation of being immediate (to the senses); Manifest, not through the appearance of a vision (of Him); Inward (batin), not through separation (muzayalah); Apart (muba'in), not through distance; Near, not through approach; Subtle, not through corporealization; Existent, not after nonexistence; Active, not through coercion; Determining, not through the activity of thought (jawl fikrah); Directing (mudabbir), not through movement; Desiring, not through resolution; Willing (sha'), not through directing attention (himmah); Grasping (mudrik), not through touch (majassah); Hearing, not through means; and Seeing, not through organs.
Times accompany Him not, places enclose Him not, slumber seizes Him not, attributes delimit Him not, and instruments (adawat) are of no use to Him. His being (kawn) precedes times (al-awqat), His existence (wujud) non-existence and His beginninglessness (azal) beginning (al-ibtida').
(Imam Ali-al-Rida (a.s.) on not delimiting God through our own limitations - from “A Shi'ite Anthology”)