You lose weight
by burning more calories (which are stored as fat) than you take in.
If you burn 1200 and take in 1200, it's a break even. If you take in 1200 but burn 2000, you're body will go to stored fat to get the additional 800. Bear in mind it takes 3500 calories to create a pound of fat, and a 3500 deficit to lose a pound.. Your goal should be slow, steady decrease.