What is meant by Intertestamental Literature?

We read in Jude 1:14-15: > Enoch, who lived in the seventh generation after Adam, prophesied about these people. He said, “Listen! The Lord is coming with countless thousands of his holy ones to execute judgment on the people of the world. He will convict every person of all the ungodly things they have done and for all the insults that ungodly sinners have spoken against him.” Footnote under the verses reads:The quotation comes from intertestamental literature: Enoch 1:9. My question is: What is meant by Intertestamental Literature ?