A "backseat driver" is a passenger in a vehicle who is not controlling the vehicle but who excessively comments on the driver's actions and decisions in an attempt to control the vehicle. The term is also used allusively for any person who intervenes with advice and instructions in affairs they are not responsible for, or subjects they may not understand well.
"Armchair professional" terms such as "armchair general" and "armchair quarterback" have similar meanings. The former is used to refer to somebody who is not in the military but seems to know better than the generals who plan military operations. The latter refers to a sports fan who seems to know better than the players themselves and is always eager to shout advice whether he/she be live at the game or more commonly sitting at home in their chair.