From Mortimer J. Adler’s classic, How to Read a Book: The Classic Guide to Intelligent Reading:
“State in your own words!” That suggests the best test we know for telling whether you have understood the proposition or propositions in the sentence. If, when you are asked to explain what the author means by a particular sentence, all you can do is repeat his very words, with some minor alterations in their order, you had better suspect that you do not know what he means. Ideally, you should be able to say the same thing in totally different words. The idea can, of course, be approximated in varying degrees. But if you cannot get away at all from the author’s words, it shows that only words have passed from him to you, not thought or knowledge. You know his words, not his mind. He was trying to communicate knowledge, and all you received was words.
To do this is not only a test of intelligent reading, it’s also necessary to intelligent conversation, especially difficult or tense conversation.