A plastic cup full of ice floating in a pitcher of beer, keeping the beer as cold as possilbe. Floaters are only used in the summer in bars without air conditioning, by patrons who are drinking slowly enough to worry about their pitcher of beer getting warm. Thus they are fairly rare.

You have to be careful about the size of the cup, the amount of ice in it, how you place the floater, and holding it while pouring beer, to ensure that it doesn't become a sinker and fill your pitcher of beer with ice.