The short book of Jude, contains many examples to illuminate the main theme of his letter (concerning ungodly people in the church). However, some of these illustrations fail to accomplish that goal for us in our world today. They are more cryptic than clarifying.
The first is in verse 9, where Jude talks about the archangel Michael having a disagreement with the devil about Moses’ body. Now we may be familiar with the angel Michael; he is mentioned in the book of Daniel and Revelation, but there is no mention in the Bible about him and Satan verbally sparring about Moses. This verse is actually a reference to an ancient, non-biblical text, called “The Assumption of Moses.”
Similarly, in verse 14, Jude mentions a prophecy of Enoch. We also know of Enoch from the book of Genesis, but there is no mention of him ever prophesying. Again, this is a reference to an ancient non-biblical text, “The Book of Enoch.”
Jude was comfortable using examples from these two books because they would have been common knowledge to the people he was writing to. As such, these familiar references would have helped readers, in that day, better comprehend the points he was making.
That is not to imply that these non-biblical books need to be elevated to the same level as the Bible or used as a viable source for forming our theology. There were merely communication tools, along the lines of Paul, in his letter to Titus, citing a local poet’s disparaging remarks about his own people of Crete.
While all these references may be confusing to us now, they were clarifying back then.