It is reasonable to assume that John himself would have given us some indication right up front where we are going, as well as a concluding statement summarizing where we have been. What we discover is that the book of Revelation is all about Christ the King. And since we know that where the King is there is the Kingdom, we may also say that the book is about Christ the heavenly King and his heavenly Kingdom. The coming of the heavenly King and his Kingdom. The cosmic oath of this coming King.
Revelation translates the Greek word apocalypse.
Hence, the book is sometimes called The Apocalypse. Basically it means an unveiling or disclosure of something that had been concealed. In this context, the thing that was once concealed and is now revealed is the reality of the risen Christ reigning as Lord in the heavenly world. For example in Gal. Paul is referring here to the brilliant and visible Christophany that shone upon him on the road to Damascus.