Simple terms: The 'reason' that to why you have to use some sort of effort to move or stop something like an object.

The law/principle of inertia states that a body at rest will remain at rest and a body in motion will continue moving with a constant velocity unless acted upon by an external non zero force.

Don't confuse this with momentum

when you can't get off the couch.

Tendency for a person to resist a change in movement; Force needed to generate the momentum to cause movement

inertia is a property of matter

Boostbuddy line of performance and making your slow car fast