All objects, if they are moving or at rest, want to stay in their state of motion. In the other word, an object at rest tends to stay at rest, and an object in motion tends to stay in motion with the same direction and speed. The resistance to the change of motion is called inertia. Newton’s first law is also called the principle of inertia. This explains, for example why you move forward in your seat of a car, when the car suddenly stops. You want to keep your state of motion , because of the inertia of your body, and you move forward