When a car accelerates, the inertia of your body resists the change in motion and tends to keep you moving at the same speed as before. This means that your body exerts a force on the seat of the car, and the seat exerts an equal and opposite force on your body, pressing you back into the seat.

Inertia is the tendency of an object to resist any change in its motion. This means that if an object is at rest, it will stay at rest, and if an object is moving, it will continue moving at the same speed and in the same direction unless acted on by an external force.

In the case of a car accelerating, the external force is the force exerted by the engine on the wheels. This force causes the car to accelerate, but your body resists the change in motion due to inertia. As a result, you feel pressed back into the seat of the car.