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.