Yep, I've got one of those somewhere, too. It either came with an LC (original) or a Centris 610, but I can't remember which.
In any case, it's like Nermal said--an adapter that lets you hook standard RCA (red/white) stereo inputs to the headphone jack on your Mac. Apple hasn't made one for a long time, but you can buy one from Radio Shack or any other consumer electronics outlet for a few bucks.
Actually, its possible it was an adapter for the audio input (stereo line-in to Mac mic jack), in which case it might have some sort of internal resistance.
It's one or the other, though.