For every action there is an equal and opposite reaction. What does these words mean in physics?

The above given statement represent the Newton; 's 3rd law of motion.

This describes that when two objects collides or interacts with each other, there occurs a pair of forces that directly acts on the two colliding objects. The force exerted on both the sides are of equal magnitude.

For example, there are two contrasting body named M and N, which collides with each other. This collision results in the occurrence of force acting on both the objects. The body M exerts a force on body N and similarly, the body N exerts a force on body M, and this two forces are equal in amount.