Newton's first law of motion states: "an object in motion stays in motion, and an object at rest stays at rest, until acted upon by an unbalanced force."
When you're riding in a vehicle, your body is moving forward. When the vehicle slows or turns, your body's inertia continues to move you forward. The seatbelt is the unbalanced force that pushes you back into your seat. Without it, you would slide off your seat.
