Your fat is stored energy, and energy is measured in calories. So say you want to loose 10lbs, you have to burn 10 lbs worth of calories more than you take in (a little more to it than that, but that's the high level). Each lb of fat has about 3500 calories, so to loses 10lbs, you have to burn about 35000 calories more than you consume. The good news is you burn about 2000 just being alive every day. So if you exercise you burn even more, watch what you eat...and you can pretty easily create a 500 or 1000 calorie per day gap.