Well, there is no inertia mass (unless you meant inertial mass) but mass can have inertia.
This is Newton's first law of motion. It states that an object at rest will stay at rest and an object in motion will stay in motion unless acted upon by an external force.
An example is when you are in a car and it stops very fast. You keep going forward a little but the seat belt stops you. So you became an object in motion and you kept staying in motion until an external force, the seat belt, acted upon you by stopping you.
This all works similarly in the world. A bullet will keep staying in motion relatively the same. But when you factor in air resistance and gravity, you see that those are the external factors. Air resistance slowly stops the bullet and gravity makes it hit the ground eventually, where it stops.