A stream of frequencies, like in a wav, mp3, wma or whatever file. A stream of sounds, so to say, as opposed to music that is generated in the sound synthesizer of a console as a list of instruments (or a mini-program that runs on the chip to generate a certain sound) and a digital equivalent of sheet music, such as midi, or the "sound fonts" used in the PS2 and also SNES or GBA music. The latter uses a lot less storage space, and is usually easier to loop, and the quality greatly depends on the power of the sound synth in whichever device. A streamed audio file always sounds the same and doesn't depend on hardware as much in order to be consistent in quality.

For example, the SNES has a much better sound chip than the GBA, so when you feed the GBA chip with the "sheet music" from FF6, the output music is of a much lower quality than it was on the SNES, and some instruments don't even sound the same. However, if the games had used mp3 or some other format like that, they'd sound almost identical without any work needed to be done to the files. However, the "sheet music" files are in the range of a 10-100 kb for a track, while an mp3 would have required several megabytes, which is kind of a problem when your game carts are like 8 MB in total.

A good analogy is realtime cutscenes versus pre-rendered videos in games.