Law of conservation of linear momentum is a extremely important consequence of Newton's third law of motion in combination with the second law of motion.
According to law of conservation of momentum When two or more bodies acts upon each other their total momentum remains constant provided no external forces are acting
So, Momentum is never created or destroyed.
When this law is applied for a collision between two bodies, the total momentum of the colliding bodies before collision is equal to the total momentum after collision
m1u1+m2u2=m1v1+m2v2 ,is known as the law of conservation of momentum