[common] Features added to a program or system to make it more flavorful from a hacker's point of view, without necessarily adding to its utility for its primary function. Distinguished from chrome, which is intended to attract users. “Now that we've got the basic program working, let's go back and add some bells and whistles.” No one seems to know what distinguishes a bell from a whistle. The recognized emphatic form is “bells, whistles, and gongs”.
It used to be thought that this term derived from the toyboxes on theater organs. However, the “and gongs” strongly suggests a different origin, at sea. Before powered horns, ships routinely used bells, whistles, and gongs to signal each other over longer distances than voice can carry.