Its pretty simple. Momentum equals the mass times the velocity(p=mv). Conservation of momentum usually has to do with collisions. In a collision, the sum of the momentum of the two objects is equal to the momentum after the collision. for example, take a bullet of mass=m and a piece of wood of mass=M. The bullet is shot at a velocity of V. after it hits the wood, the wood and the bullet move together at a velocity of v.
conservation of momentum states that Pinitial=Pfinal, or mV=(m+M)v