Slang. A
person whose main
leisure activity is watching
television.
"Couch" refers to the furniture the person sits on to watch television, and "potato" is a metaphor which likens the person to a tuber, which has eyes, does not move, and tends to have a round shape. Extended television viewing requires eyes, but not locomotion nor use of arms and legs, and the lack of exercise involved may tend to make the person in question potato-like in shape.