Metabolism is the burning of calories necessary to supply your body with the energy it needs to function. Whether you’re sleeping, eating, drinking, cleaning etc. Your body is constantly burning calories to keep you going. It’s affected by your body composition – meaning the amount of muscle compared to the amount of body fat you’ve in your body. Muscle uses more calories than fat. If you’re more muscular and have a lower body fat percentage, you likely have a higher metabolism than say your friend, who’s less muscular.