The difference between "disc at once" and "track at once" when it comes to burning an audio CD, simply boils down to a 2 second pause between tracks. Burning "disk at once" will burn all of your audio tracks, with no 2 second pause between them.
Now, if you're just copying a disc, it may or may not matter because your software might do disc at once even if the option isn't checked. You can verify this by, you guessed it, checking for an extra 2 seconds of silence between tracks.
The 2 seconds usually isn't a big deal, except in cases where one track is supposed to flow into another...in that case it's rather annoying.
Slackmaster 2000