"Calories are a measure of the amount of energy in food. Knowing how many calories are in our food can help us to balance the energy we put into our bodies with the energy we use. And that's the key to a healthy weight."
Women and men have a daily amount of calories they should take in. The average for a woman is 2000 calories and a man's is 2500 calories. This also depends on your activity level; more physical activity the more calories you need (the more energy you need).
Different sorts of food has different amount of calories. For instance a meal at McDonalds:
1 Big Mac (214 g) = 490 kcal
Regular Fries (78 g) = 225 kcal
0.5 L coca cola = 215 kcal
Together it equals 490+225+215=930 kcal. In other words; by eating a (regular) meal at McDonalds you have, if you are a woman, consumed half of the daily amount of calories you need. The math in keeping healthy is easy: if you eat more (calories) than you need, you'll put on weight. If you eat too little (calories), you'll lose weight.
It is also really important to remember that skinny doesn't necessarily mean healthy. It is important to get all the calories your body needs to function and also to eat varied and a wide range of foods to ensure that your'e getting a balanced diet and that you body is receiving all the nutrients it needs.