What does the conversation of momentum refer to?

The conversation of momentum is: When a system is closed, the total momentum before an event is equal to the total momentum after an event. E.g. A van travels towards a stationary car. Because the van is moving, it has momentum in the forward direction. The momentum of the van is its mass x velocity. While the stationary car has no momentum and is not moving. But if the van collided into the car, both the van and the car would move together at a lower velocity than the initial velocity of the van. In this system of the van and car, the total moment is now the same as the initial momentum of the van alone. Therefore the total moment before the collision is the same as the total momentum after the collision. Ergo, momentum has been conserved.