What is a back seat driver?

A back seat driver is someone who criticizes or gives unwanted advice to the person who is driving. This person is not usually in control of the vehicle, but rather sitting in the back seat, and offers constant commentary on the driver's decisions and actions. Back seat drivers are often seen as annoying and bothersome, as their input can distract the driver and cause tension in the vehicle. At times, their comments can be helpful, but most often than not, they are not needed and disrupt the driving process.