"Action and Reaction" refers to Newton's Third Law of Motion, which states that for every action, there is an equal and opposite reaction. This means that when one object exerts a force on another, the second object exerts a force of equal strength in the opposite direction. For example, when you push against a wall, the wall pushes back with the same force. This principle can be observed in everyday life. When a rocket launches, the engines push down on the ground, and in response, the ground pushes the rocket upward. This interaction is crucial for understanding how forces work in our physical world.