A bodies reluctance to move or change direction

Inertia is pretty cool.

The larger an object is the more inertia it will have.

If you imagine a stationery bus, its quite large and would be very difficult to push along the road.  This is because it has a large moment of inertia.

A small object will have a small amount of inertia, and its easier to push a car than a bus.

Inertia in trampolining helps us control the speed of rotation.  If we are rotating too fast, we can open our body out to make a larger shape, increasing the moment of inertia and slowing the speed of rotation.