Calories are a measure of the amount of energy in food. We measure the amount of energy contained in an item of food in calories (kcal).
When we eat and drink, we’re putting energy (calories) into our bodies. This energy is used by our bodies, and the more physical activity we do, the more energy (calories) we use.
To maintain a stable weight, the energy we put into our bodies must be the same as the energy we use by normal bodily functions and physical activity.
If we regularly burn more energy that we consume, this would lead to weight loss. Weight gain occurs when we regularly put more energy into our bodies than we use.