Conservation of linear momentum (COLM) is applicable to a system experiencing no net external force.

It says that internal forces alone cannot change the momentum of the system. In other words, the momentum of the system is conserved in the absence of external forces.

A typical example is that of a bullet fired from the gun. Initially, the bullet is inside the gun and the momentum of the system (gun + bullet) is zero. Now, when the gun fires the bullet, the bullet has forward momentum. However, the forces between the bullet and the gun are internal forces. So, the overall momentum must be zero later as well. So, the gun moves backwards and has backwards momentum. This ensures the overall zero momentum of the system.