In simple terms, a calorie is a unit of energy. The more calories you eat, the more energy you have. The trouble is that if you can not use burn of the energy you have eaten through your normal daily activities, then the left over calories (or energy) will turn into fat. The average man should aim to eat no more than 2500 calories per day and a woman should eat no more than 2000 calories per day. Once more than this is eaten, then it will need to be used up through activity or it will turn to fat.