Inertia is basically,

"An object at rest will stay at rest or an object in motion will stay in motion unless acted upon by an external force"

For example:

" A soccer ball being kicked by a soccer player. "

The soccer ball was first at rest and then it was kicked by the soccer player allowing it to move in motion and it will continue to what direction it was kicked towards unless it was acted upon by an external force like friction, gravity, etc.