If one object exerts a force on a second object, the second object exerts a force equal in magnitude and opposite in direction on the object body. This is also known as the law of interaction.
action and reaction are equal and opposite
states that for every action, there is an equal, and opposite, reaction. E.g. when you jump into the air, there is a force equal and opposite to the one that moved you upward. The reason the Earth doesn't seem to move away from you is that it is large enough that your pushing it had almost no affect
In a system where no external forces are present, every action force is always opposed by an equal and opposite reaction.
Rockets are based on this principle. The third law states that for every action there is an equal and opposite reaction.