Momentum

If something is moving, it will keep going in the same direction, and at the same speed unless it is changed by a force, such as someone pushing or pulling it.

This is called momentum

The momentum of an object depends on its speed and mass, such that fast, heavy objects (such as Nelly on her skateboard) will have more momentum than slow, light ones.

Things with a lot of momentum are much harder to stop - hence Nelly's concern.