This book describes the fall of the Watchers, the angels who fathered the angel-human hybrids called Nephilim.
The remainder of the book describes Enoch's revelations and his visits to heaven in the form of travels, visions, and dreams.
It‘s a fascinating book with all kinds of interesting insights and some of the things that we think might be in the Bible are actually in the Book of Enoch, things about the names of certain angels and whatnot.