First, Rosencrantz is referencing the questions game. Rosencrantz says that Hamlet "murdered" them at the game, meaning that he easily beat them.
The line, of course, has a deeper, ironic meaning -- one that the audience would recognize. Rosencrantz and Guildenstern will die, thanks to Hamlet -- making Rosencrantz's line an example of both foreshadowing and irony.
