"An alleged scientific discovery has no merit unless it can be explained to a barmaid." – Lord Earnest Rutherford of Nelson
How well do you really know something you're learning about? Fairly well? Great. Explain it to someone else.
Taking what you know and explaining it to another person is a great way to determine if you know something, or if you know of something. Did you memorize the textbook? Or do you truly understand how it all works?
While you're in school you might be tempted to simply memorize the textbook. After all, that's what you're graded on: how well you can regurgitate what you had read. The problem with that grading style is that it simply tests your ability to memorize, not to actually do anything with that knowledge.
Which one is more valuable to you: the grade or the knowledge?