To depend is to have confidence in something or someone. You can depend on your most loyal friend to show up for your modern dance performance in the park (although you probably can't depend on her liking it).
Depend can also mean "determined by or contingent on something else." For example, if rain is in the forecast, your outdoor dance performance may be cancelled; it depends on the weather. Depend is almost always followed by the word on or upon, with upon being more formal. If you're speaking informally, you may drop the on or upon, as in "It all depends how you feel." Your decision to do that will, of course, depend upon your audience.