What is meant by recoil in a gun?

recoil is the backward momentum of a gun when it is discharged.
Remember that, for every action, there is an equal and opposite reaction. So, you have a force blasting forwards, and it's going to push back in the opposite direction. That's recoil.
Recoil is the force exerted on the operator of a firearm as a result of the firearm's operation.

The Laws of physics dictate equal and opposite forces for a reaction. The same force which accelerates the bullet creates an accelerative force on your body when you shoot. While you have the weight of the firearm plus your own body, and thus experience very little movement; the far lower mass (and corresponding resistance to acceleration) of the bullet results in said bullet gaining a great deal of speed and departing the barrel of the gun.