Signs for us to know when the time is near. The
three woes.
Simplistic version: Three succesive sets of 7 judgements. The completion of one set "appearing" to precede the starting of the next set.
The seal, trumpet, and bowl judgements each progressively worse than the last set.
After the last bowl judgement in chap 16, chapters 17-22 Give a summary of things spanning the previous few thousand years, and future past the millenial reign and the final judgement. Some things are explained in more detail here than you will find elwhere.