Calories are a way of measuring energy - and in the situation that we have here - the amount of energy that your body gets from eating that food you stuff your face with - yum, yum. Your body needs a certain amount of energy every day to keep it going - things like keeping your muscles moving, your heart beating, your chest breathing, your body heated and your brain ticking. The energy for that comes from the food that you eat, or stuff that you drink. If you eat and drink just the right amount (with the right amount of calories in them) to move your muscles, work your brain etc then you won't get too fat or too thin. Eat too many calories in your food and your body turns it into fat - and so your tum and bum get gross; eat too few and you begin to disappear.