Momentum is a measure of how hard it is to stop something. Something heavy moving at a fast speed has a lot of momentum. For example the truck in the picture above. If there was a car next to it and they were both rolling down the hill, the truck would go faster because it has a larger mass which gives it a greater momentum than the car. If both the car and the truck apply the breaks in order to stop it would be much harder for the truck to come to a stop compare to the car.

Evgenija Momentum