Here's what that means:

* Momentum: Momentum is a measure of an object's mass in motion. It's calculated as mass (m) multiplied by velocity (v): p = mv.

* Closed system: A closed system is a system where no external forces act upon it.

* Constant: The total momentum of the system doesn't change, even if objects within the system interact with each other.

In simpler terms: Imagine a collision between two objects. Even though their individual momenta change during the collision, the sum of their momenta before the collision equals the sum of their momenta after the collision.

Here are some key implications of the law of conservation of momentum:

* Rocket propulsion: Rockets work by expelling mass (fuel) in one direction, creating an equal and opposite momentum that propels the rocket forward.

* Collisions: In collisions, momentum is transferred between objects, but the total momentum of the system remains constant.

* Explosions: The fragments of an explosion move in different directions, but their combined momentum is equal to the momentum of the object before the explosion.

The law of conservation of momentum is a fundamental principle in physics and plays a crucial role in understanding many physical phenomena.