My understanding, though I am not professional, only a hobbiest, is that when an album is recorded they have a tape (track) of each intrument that is used in the song. When they put the album together they line up the tracks and combine them into one track which becomes the master tape which they cds/cassettes/vinyls are recorded from.
The multi-tracks Les is referring to is the tracks of all the different intsruments. Without them the dvd audio can not be produced.
Like in the Rumours DVD-Audio, they took each track and cleaned up all the debris and excess nosie that were picked up to give it a clear/clean sound, so when you listen to the DVD-Audio, you can really hear all the details in those songs. Things that you can't pick up off a regular CD or cassette.
Most major studios are 64-track studios or in some cases even more, which means they can produce songs with up to 64 different instruments or elements. Like one for stevie's vocals, on for chris's, on for lindseys. and then one for each back up vocal, one for each insturment line, ect.