A table of nutritional facts with calories listed underneath
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.

How many calories do you need?

Calorie table with figures for boys and girls at different ages.
Recommended daily calorie intake for boys and girls at different ages

If you're calorie counting then in order to lose weight, you first need to find out how many food calories you, your very own self, need on a daily basis in order to lose weight. This will be different from person to person depending on things like how old you are and whether you're a girl or a boy.