First of this should probably be under physics and not biology... nevertheless for your question. Newton's first law basically says that an object in motion will stay in motion unless acted upon by another force. Initially, you are traveling at the same velocity as your car, however if the car suddenly stops and you are not wearing a seat belt you will continue to move forward at the same velocity until you smack against the steering wheel or dashboard. A seatbelt basically connects you to the car so when the car suddenly stops you stop by hitting into the seatbelt which would prevent the injuries of hitting the dashboard or breaking and flying through the windshield.