It is inertia, so it is not really a force but rather the absence of force. When the car accelerates, your body need to accelerate as well, and therefore your seat has to apply a force to your body. The parts of your body that are not in contact with the seat do not experience this force, and therefore they are not accelerated at first. But relative to your body, it feels as if they were being accelerated backward by a force. It is called a fictional force because it is not real.