As Newton's First Law of Motion states, an object in motion wants to stay in motion.
When you are riding in a car, both you and the car are in motion. When the driver steps on the brakes, the brakes create friction with the car's tires, causing the car to slow down.
Usually, if the car slows down gradually, you'll slow down with it. The friction of your legs against the seat, the seatbelt holding you down, your feet pressing against the floor, and other things combine to help you slow down too.
But if the car stops suddenly, inertia will cause you to keep going forward unless you are somehow tied down, like with a seatbelt.
So, because of inertia, it's important to wear your seatbelt when you ride in a car, to make sure that you stop when the car stops.
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Parsons
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inertia and Newton's First
Law of Motion.