When you fire a bullet from the gun, the bullet travels in the forward direction with a very high velocity.
On the other hand, the gun also experiences a force that is in the backward direction.
This is known as "recoil".
Now the question may arise why the bullet travels so fast whereas the gun doesn't?
To understand this, recall that momentum is the product of mass and velocity.
The momentum of the gun must be equal to the momentum of the bullet.
The bullet has a smaller mass but greater velocity.
The gun has a greater mass but smaller velocity.
The end result is that the momentum of both gun and bullet is equal.