The phrase "For every action there is an equal and opposite reaction" is a statement of Newton's Third Law of Motion. It means that whenever one object exerts a force on another object, the second object simultaneously exerts a force equal in magnitude but opposite in direction back on the first object. This expresses the fact that forces always come in pairs, known as action- reaction pairs, and one cannot occur without the other. For example, when a ball hits a wall, the ball pushes the wall, and the wall pushes back on the ball equally in the opposite direction, causing the ball to bounce off. This law is fundamental to understanding interactions in physics and is closely related to the conservation of momentum.