Game used cards have memorabilia attached to them that was worn in a game. In hockey cards they have cards with pieces of jersey, logo patches, small pieces of skates, gloves, sticks, fight straps.....Even netting from the goal.
To be sure you are getting game used memorabilia though, you have to deck the back of the card as it will specifically say whether it's game used, or a piece of manufactured memorabilia that was not game used.