What is inertia?

The attempt of a body to retain its current state of movement is called inertia. A body retains its state of rest (the body does not move) or its current movement (i.e. a linearly uniform movement) until an external force will act on it.

A dimension of the inertia of a body is the heavy mass of a body with linear movements (translation) and its moment of inertia with rotational movements (rotation).

The more heavy mass a body possesses the more inert it is and the greater force has to be exerted to change its state of movement.

Example:

To move a toy car it is sufficient to push it with your finger.
To move a proper car, you have to exert all your power yet.

You can see from the red ball in the video that it rolls on in the driving direction while the car is braking because of its inertia.