A phrase used directed towards a person who doesn't do hardly anything. It is directed towards lazy people who don't do much at all, but still need to do less. It is very appropriate to remind him/her that she needs to do less.
Matt: Hey, what's up Mark Jay, what are you up to right now.
Mark: Well, I'm just sitting in my desk chair, on the computer and watching Duke lose in every sport.
Matt: Do less.