A theorem that's in the top five of most useless things you'll learn (or not) in calculus. Unless your teacher tells you it's on the test. (And there may be a multiple choice question on the AP if you're taking that.) But even then you need to ask your teacher to get very specific about what problems you're supposed to be able to work with this thing, because it's never been obvious to me.
If you do not have an account, you should get one, because it is awesome! You can save a playlist for each test or each chapter, and save your "greatest hits" into a "watch right before the final" list (not that we recommend cramming, but when in Rome...)