Something depreciates when it loses value. If you bought shares of a company for $10 each a year ago and now they’re worth $1 each, guess what: they have depreciated.
One of the most frustrating things about buying a car is the way it loses value. As soon as you drive it off the lot, your new car has already depreciated by several thousand dollars. Though usually used like this as a financial term, depreciate can also be used as the similar-sounding but unrelated word deprecate meaning "to belittle, lower in esteem." Your teachers should not depreciate you just because you don't always know the answers to their questions!