The Book of Enoch is any of several pseudepigraphal (falsely attributed works, texts whose claimed authorship is unfounded) works that attribute themselves to Enoch, the great-grandfather of Noah; that is, Enoch son of Jared (Genesis 5:18). Enoch is also one of the two people in the Bible taken up to heaven without dying (the other being Elijah), as the Bible says "And Enoch walked with God, and he was not; for God took him." (Genesis 5:24; see also Hebrews 11:5). Most commonly, the phrase "Book of Enoch" refers to 1 Enoch, which is wholly extant only in the Ethiopic language.
The biblical book of Jude quotes from the Book of Enoch in verses 14-15, “Enoch, the seventh from Adam, prophesied about these men: ‘See, the Lord is coming with thousands upon thousands of his holy ones to judge everyone, and to convict all the ungodly of all the ungodly acts they have done in the ungodly way, and of all the harsh words ungodly sinners have spoken against him.’” But this does not mean the Book of Enoch is inspired by God and should be in the Bible.
Jude’s quote is not the only quote in the Bible from a non-biblical source. The Apostle Paul quotes Epimenides in Titus 1:12 but that does not mean we should give any additional authority to Epimenides’ writings. The same is true with Jude, verses 14-15. Jude quoting from the book of Enoch does not indicate the entire Book of Enoch is inspired, or even true. All it means is that particular verse is true. It is interesting to note that no scholars believe the Book of Enoch to have truly been written by the Enoch in the Bible. Enoch was seven generations from Adam, prior to the Flood (Genesis 5:1-24). Evidently, though, this was genuinely something that Enoch prophesied—or the Bible would not attribute it to him, “Enoch, the seventh from Adam, prophesied about these men” (Jude 1:14). This saying of Enoch was evidently handed down by tradition, and eventually recorded in the Book of Enoch.
We should treat the Book of Enoch (and the other books like it) in the same manner we do the other Apocryphal writings. Some of what the Apocrypha says is true and correct, but at the same time, much of it is false and historically inaccurate. If you read these books, you have to treat them as interesting but fallible historical documents, not as the inspired, authoritative Word of God.