Newton's Third Law of Motion is that all forces operate in equal pairs that move in opposite directions. This is often summarized as "for every action, there is an equal and opposite reaction."
This law means that when one object exerts force on another, the same amount of force is exerted on the initial object, but in the opposite reaction. For example, when a billiard ball strikes another ball, the second ball is propelled forward. Simultaneously, the momentum of the first ball is slowed or stopped by opposing force. The amount that the first object is affected by the opposing force depends on the mass and motion of the second object.