Inertia:

The inertia of a body is its property due to which it any change in its state of rest or motion.

Galileo related the inertia of a body with its mass, the greater the mass of a body greater its inertia.

Inertia ∝   mass of the body

Experiment:

Take a glass and cover it with a piece of cardboard. Place a coin on the cardboard. Now kick the card horizontally, with a jerk of your finger.

The coin does not move with the cardboard due to inertia. The coin falls into the glass as the card flicks away.

Experiment:

Cut a strip of paper. Place it on the table. Stack a few coins at one end. Pull out the paper strip under the coins with a jerk.       

Coins stacked over remain undisturbed on pulling the paper strip quickly

Coins stacked over remain undisturbed on pulling the paper strip quickly due to inertia.