You burn about 2000 calories a day (depending on your gender, size, etc) doing absolutely nothing. Your body's metabolism burns calories. If you naturally burn 2000 a day, and then through exercise burn another 300 and only eat 2000 calories, you will be in a calorie deficit.

In short, track what you eat and don't overeat. It's not complicated, despite what all the fad diets and weight loss companies would lead you to believe.