Seat belts attach your body mass to that of a car, meaning that when the car accelerates or decelerates, you do also. Were it not for seatbelts, your body would be acted upon by Newton's First Law independently of the vehicle.
The basic idea of a seat belt is very simple: It keeps you from flying through the windshield or hurdling toward the dashboard when your car comes to an abrupt stop. But why would this happen in the first place?
Seat belts protect vehicle occupants by decreasing the time it takes them to come to a stop in a crash, spreads the impact force over a greater area of the body, minimises contact with the interior of the vehicle and helps stop them from being ejected from the vehicle.
For adults and older children (who are big enough for seat belts to fit properly), seat belt use is one of the most effective ways to save lives and reduce injuries in crashes.