Newton's Third Law of Motion states that for every action, there is an equal and opposite reaction. It describes how when one object exerts a force on a second object, the second object exerts a force back that is equal in magnitude and opposite in direction. The document provides examples of this law, such as a person jumping out of a boat causing the boat to move backwards with an equal force, or a person pushing down on a diving board causing the board to push them up with an equal opposing force.