Hello!
So basically,
The law of Conservation states that,
When two bodies collide with each other in the absence of an external force, then the total final momentum of the bodies is equal to their total initial momentum. '
Let's take an example,
There is a Balloon, particles of gas move rapidly colliding with each other and the walls of the balloon, even though the particles themselves move faster and slower when they lose or gain momentum when they collide, the total momentum of the system remains the same.
Balloon will not change it's size untill an external force is not applied. If we provide heat to it the balloon will expand.