When this is said, it is almost always referring to a fantastic organ piece written by J.S. Bach. It's often used (or imitated) in horror movies to set a scary mood, although usually only the first minute or so is used. It was in Fantasia. If none of that helps you, try this: it goes something like "Da da da.......da-da-da-dum--da-//long pause\\-Da da dum......da-da da-dum", repeats that a couple times, and then builds to a crescendo. If none of those things helps bring it to mind, you should really find it and listen to it, because it's a pretty amazing piece of music.

Besides being the best composer of his era, Bach was also one of the best organ players. As you might expect from the title, the piece has two parts: a toccata and a fugue (well, duh). The point of the toccata is to show off a musician's virtuosity. So it's a big, showy part with lots of crescendos and decrescendos and complicated chords and the like. The fugue is a complex musical form that more qualified noders have already expounded upon. Suffice it to say that Bach could write a kick-ass fugue. And this one is no exception.