Let’s see how simple we can make things…
The definition of momentum in physics:
Momentum describes how strong a moving thing is. Things that aren’t moving have no momentum. Moving things have less momentum if they are light or moving slowly, and more momentum if they are heavy or moving fast.
Because of momentum, a planet or a star that is moving in space will generally keep moving in the same direction unless something happens to push it another way. The Sun keeps going around the Milky Way, and the Earth keeps going around the Sun. But on Earth, friction with the ground or the air will gradually push an object and make it stop.
One way to think of momentum is that momentum measures how hard it will be to stop the object. An object has more momentum if it is bigger, or if it is going faster – momentum is equal to mass times velocity (speed).
It’s a real thing, people…