Antilegomena means "spoken against" and refers to books in the Bible that some people didn't agree should be included. Imagine you are playing with a group of friends and you suggest playing a certain game, but some of your friends say they don't want to play it. This is kind of like what happened with the books of the Bible. Some people thought they shouldn't be included because they weren't sure if they were really written by the people they were attributed to, or if they contained correct teachings.
There are different books that are considered antilegomena depending on the branch of Christianity. For example, some Protestant Bibles include books in the Old Testament that are considered antilegomena by the Catholic and Orthodox Churches.
However, just because a book is considered antilegomena doesn't mean it's not important or useful. People can still learn from them and read them as part of their spiritual practice. It's just that some people don't think they should be included in the official canon of the Bible.