The revelation of Jesus Christ which God gave to him to show his bond-servants the things that must happen soon. He made it known when he sent it by his angel to his bond-servant John, who bore witness to the word of God and the testimony of Jesus Christ, to as many things as he saw. Blessed is the one who reads aloud and blessed are the ones who hear the words of the prophecy and keep the things written in it, for the time is near. [Translation by David Mills.]John identifies this document as a revelation of Jesus Christ, which suggests that in it Jesus will fully disclose something. That which he disclosed he also made known; that is, he explained it and made it clear  when he sent his angel to John who then bore witness to it. In this way, Jesus was able to show it to his bond-servants. To show simply means to present something to human senses so that it can be known. Hence, those who heard the Revelation would expect to have Jesus make something known to them in a clear way so that they could understand it.
So, what is to be revealed? John says it is the things that must happen soon. John reemphasizes that the things revealed are near when he says the time is near at the end of the prologue. Because the things that must happen will happen in the near future, a degree of urgency is created for hearing them and understanding them. These are not things in the distant future. These are things that those who originally received the message would experience.
John gives a clue to the kinds of things revealed when he writes the things that must happen soon. These words allude to Daniel 2:28 (LXX) where Daniel told Nebuchadnezzar that God had revealed to him "the things that must happen in the latter days." Because John replaces "in the latter days" with "in a short time," meaning soon, he suggests that the latter days have arrived when God would set up a kingdom which would fill the whole earth (Daniel 2:44). John was a companion with his fellow bond-servants in the kingdom (Revelation 1:6, 9). Soon that kingdom would be filling the whole earth. Again, the message is urgent because those receiving the message are called to participate in the conquest and become conquerors or victors.
 BDAG, s.v. ἀποκαλύπτω and ἀποκάλυψις; Louw and Nida, #28.38. In the NT, a revelation is always a divine disclosure. God/Jesus either reveals or is revealed. Here Jesus reveals not so much himself as the things that must happen soon.
 BDAG, s.v. σημαίνω; Louw and Nida, #33.153. The focus of this word seems to be on the process of making something clear or understandable.
 BDAG, s.v. δείκνυμι; Louw and Nida #28.47. The context of this verb almost always mentions the person who receives the message, so the word seems to focus on the transfer of the message from the sender to the receiver.
 For an explanation of why this clause does not mean "things that must happen quickly," see G. K. Beale, The Book of Revelation: A Commentary on the Greek Text, New International Greek Testament Commentary (Grand Rapids, MI; Carlisle, Cumbria: W.B. Eerdmans; Paternoster Press, 1999), 182. Alan F. Johnson, "Revelation", The Expositor's Bible Commentary, Volume 12: Hebrews Through Revelation, ed. Frank E. Gaebelein (Grand Rapids, MI: Zondervan Publishing House, 1981), 416.