Newton's First Law states that an object at rest remains at rest and that an object in motion tends to stay in motion.
Example: A sofa that is sitting in a living room stays at rest by remaining still.
A car that is in motion stays in motion by continuing on its course.
If an object isn't moving, it won't move unless a force acts upon it. If an object is moving at a constant velocity it will continue at a constant velocity, unless it is acted upon by a force.
All objects resist change to their motion. The resistance an object has to the change of its motion is called inertia.
Example: When you are riding in a car and you are forced to stop suddenly, inertia keeps you moving forward. In a car accident a seatbelt is designed to change your motion and prevent you from moving forward.
The amount of inertia an object has depends on its mass. The greater the mass of an object, the more force required to change its motion.
Example: A semi-truck loaded with bricks has greater inertia than a 4-door car. It takes more force to stop the movement of a loaded semi-truck than a 4-door car.