Well, inertia is an object or body to remain inert. In other words, an object or body at rest to remain at rest unless acted upon an outside force. The same goes for an object or body in motion to remain in motion unless acted upon an outside force. Here's an example, have you ever been in an elevator? You know that feeling you get when it stops at a floor, it slightly goes up, then down fast, thats an inertia.