To be able to understand the significance of the calorie, energy metabolism needs to be understood.
In the context of the human body, the term "metabolism" refers to the way in which the body converts the locked-up energy in food to a useable form. Since the human body is incapable of utilising the energy from the earth's primary energy source, the sun, the sun's energy is indirectly extracted from plants and animal products.
This chemically bounded energy is made available to the body through the metabolism of food - the combustion or "burning" of carbohydrates and fats in the presence of oxygen. For the combustion to take place, stores of potential energy sources must be available.
For this reason, the body has several mechanisms of stocking energy, making it easily accessible for normal, day-to-day functions and saving it for leaner times.