Newton's First Law of Motion

Newton's first law of motion states that an object will remain at rest, or continue to move at a constant velocity, unless a net force acts on it.

The term 'inertia' is s shorthand for 'the principle of inertia' as described by Newton in the full text of Newton's First Law of Motion which simply expressed means that an object that is not subject to any outside forces will move at a constant velocity, covering equal distances in equal times along a straight-line path - it will remain in its state of inertia.

On the surface of the Earth the nature of inertia is often masked by the effects of friction wich makes people (among them a lot of physics students!) believe (as Aristotle did) that if something is moving there must be a force acting - regardless of whether that movement was constant or not!

You need to understand that an object that is stationary or moving at a constant velocity has no net force acting on it - no acceleration - no force.

If you are travelling in a car without a seatbelt on and the car crashes.... you will continue to move at the speed the car was travelling at - until some force makes you stop. See these commercials -

See terminal velocity and friction.