To be completly terrible at something, to not excel in any form. It may refer to someones skill at a certain subject, action, sport, or game. However it is often used to describe someones total lack of skill at games such as Madden 12, call of duty, or possibly mario cart. This phrase is also comparable to Ass Milk and Trash Sauce.
Robert: Dude, your getting your ass kicked.
Brad: Its because Tim Tebow is a shit quarterback you ass hole.
Robert: Dont make excuses man, your just Garbage Poop.