The Law of Conservation of Momentum states that in a closed system, the total momentum remains constant if no external forces act on it. Momentum is the product of an object's mass and its velocity. This means that when two objects collide, the total momentum before the collision equals the total momentum after the collision. For example, when two billiard balls collide on a table, the momentum they share before the impact is transferred between them. If one ball slows down, the other speeds up, ensuring that the overall momentum of the system remains unchanged.