For every action there is an equal and opposite reaction. What does this phrase mean in Physics?

The Term "For every action, there is an equal and opposite reaction" is derived from Newton's third law of motion.

This can be explained by an example of a block placed on the flat surface.

The weight of the block applies an action force on the surface while Surface applies a reaction force on the block. Action and reaction force applies to different bodies other would have canceled each other. Action and reaction forces are always acting in pairs. The magnitude of reaction force depends upon the action forces as both of them are equal in magnitude but opposite in direction.