Here's a breakdown:

* Momentum: A measure of an object's mass and velocity. It describes how much "motion" an object has.

* Closed System: A system where no external forces act upon it. Think of a perfectly isolated system like a sealed box.

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

Here's a simple example:

Imagine two billiard balls on a frictionless table. Ball 1 is at rest, and Ball 2 is moving towards it. When they collide, Ball 2 transfers some of its momentum to Ball 1. Ball 2 slows down, and Ball 1 starts moving. However, the *total* momentum of the two-ball system remains the same.

Key points:

* The law of conservation of momentum is a fundamental principle in physics.

* It applies to all types of interactions, including collisions, explosions, and gravitational interactions.

* This law has numerous applications in fields like rocket science, particle physics, and even everyday life.

Let me know if you'd like more details or examples!