(pardon the tl;dr)
A discrete track is an individual recorded track. This term is used in the context of multitrack recording, which is what is done in music studios. For example, vocals would be on one discrete track, guitars on another, etc.
Some versions of Audacity have multitrack recording ability within the same file, to simulate a multitrack recording studio. I would guess that mchalebk mentioned discrete tracks because the default export for Audacity assumes you want any and all discrete tracks mixed down to one stereo AIFF or WAV file. If you want to preserve the ability to manipulate those individual tracks inside GarageBand, you will have to export each of those tracks separately.
However, if all you're doing with Audacity is on one stereo track, then export the whole file to a stereo AIFF file, and import that AIFF into GarageBand.