What is Newton's third law of motion?

Newton’s Third Law of Motion

This law states that when two bodies interact, they apply forces to one another that are equal in magnitude and opposite in direction. This law is also known as the law of action and reaction. 

In other words, for every action, there is an equal and opposite reaction.

For example- If a bird exerts a force in a downward direction by pushing the air by its wings then the air pushes the bird in the upward direction with the same equal forces that was exerted by the bird in the downwards direction.