Newton's Third Law of Motion states that for every action, there is an equal and opposite reaction. This means that when one object exerts a force on another object, the second object exerts a force of the same magnitude but in the opposite direction on the first object. This principle helps explain how objects interact with each other in various situations. For example, when you push against a wall, your hand applies a force to the wall, and the wall applies an equal force back on your hand. This law is fundamental in understanding the behavior of objects in motion and is a key concept in physics, attributed to Sir Isaac Newton.