A joule is a unit of energy in the International System of Units (SI). It measures the amount of energy transferred when a force of one newton moves an object one meter. Joules can also describe energy in various forms, such as heat, electricity, and mechanical work. In practical terms, one joule is equivalent to the energy needed to lift a small apple (about 100 grams) one meter against the force of gravity. Understanding joules helps in fields like physics, engineering, and nutrition, where energy consumption and transfer are important concepts.