When you ride in the backseat of a car, you sit in the row of seats behind the driver. Kids sometimes fight over the front seat, not wanting to sit in the backseat.
You can travel in the backseat of a automobile, unless you're the driver or are riding in a two-seat sports car. Another way to use the word backseat is to describe "an inferior position." If you take a backseat to your older brother, it means he tends to get all the attention. The word, more commonly written as back seat, was used in the 1800s to talk about riding in coaches and carriages.