Simple Definition:

Metabolism is the process by which the body converts food and drinks into energy [the power to do work]. It also involves the breakdown of substances in the body to build and repair tissues and organs. For example, when you eat an apple, your body uses metabolism to break down the carbohydrates, proteins, and fats in the apple to produce the energy needed for your cells to function.

Very Simple Definition:

Metabolism is the way our bodies turn food into energy. For example, when we eat an apple, our bodies use metabolism to change the apple into the energy that helps us move and stay alive.