In order to do a comprehensive study of the book of Revelation, we must study it together with the Old Testament book of Daniel. In Daniel's day, the prophecies of the end times were sealed. Following the prophetic revelations made to Daniel while in exile in Babylon, the Lord said, "But you, Daniel, roll up and seal the words of the scroll until the time of the end." (Daniel 12:4a) The book of Revelation unseals the book of Daniel. When the Lord tells John of things to come, He says, "Do not seal up the words of the prophecy of this scroll, because the time is near." (Revelation 22:10)
In Daniel's day the end times were far off. It was impossible for Daniel to understand the prophecies of the end times prior to Christ's redemptive death on the cross. But by the time John writes the book of Revelation, the Old Testament prophecies concerning the Messiah have been fulfilled in the person of Jesus of Nazareth, and John is able to understand the visions he is given of the end times. Ever since Jesus ascended to heaven we have been essentially in the times of the end. The clock of this world is winding down. This is why John says in the first sentence of Revelation, "The revelation from Jesus Christ, which God gave Him to show His servants what must soon take place." (Revelation 1:1a) The book is unsealed because it must soon take place.