Although you can use the expression couch potato to simply mean "idler" or "layabout," it usually refers specifically to someone who watches a whole lot of TV. It was first used by a 1970's comics artist who drew lazy, sedentary characters he called couch potatoes. From there, the phrase became an extremely popular way to talk about someone who's spent so much time in front of the TV that he seems more like a vegetable than a human being.