I've written both here and in my book that teaching a concept is the best way to make sure you understand it. Whether you're studying history, chemistry or Spanish, if you can stand up in your room and explain the concept to an imaginary class, odds are that you understand what you're talking about. Teaching it makes it an active process rather than just passively reviewing the material.
Then I recently read this quote attributed to Albert Einstein:
"If you can't explain it simply, you don't understand it well enough."
That's a concept to keep in mind if you take this advice about teaching something to help you learn it. If you can make something complicated a lot less complicated, and teach other people to understand it, then you've really mastered whatever you're trying to learn.