"For every action, there's an equal and opposite reaction"

Basically, what it's saying is forces always comes in pairs. For every force, there's an equal force in the opposite direction.

The force pushing air out of a balloon has an equal and opposite force that pushes the balloon forward.

The force from a boat's propeller pushing water backwards has an equal and opposite force that pushes the boat forwards.

The force that shoots a bullet out of a gun has an equal and opposite force that causes the gun to recoil.