Inertia is a fundamental property of matter. It's the tendency of an object to resist changes in its state of motion, whether it's at rest or in motion. This means:

* An object at rest wants to stay at rest. You need to apply a force to get it moving.

* An object in motion wants to stay in motion. You need a force to slow it down, stop it, or change its direction.

The amount of inertia an object has is directly proportional to its mass. The more massive an object is, the harder it is to get it moving or stop it.