Your body needs energy to keep moving. It gets the energy that you need from calories from food. The calories that you eat are then converted to physical energy that fuels your every movement. Those that were not used are stored in the body as fats.
Weight management is all about balancing the calories that you consume with those that you burn. If your food intake is greater than what you burn then you gain weight. If you eat less, then you lose weight.
As a general guideline, 3,500 calories is equal to 1 pound of fat. So, if you want to lose one pound, you need to burn at least 3,500 calories.