HD audio is a generic term for any of the new formats found on Blu-Ray (and old HD-DVD) discs. It's a form of lossless compression for movie soundtracks. Dolby TrueHD and DTS HD Master Audio are two of the main candidates. They benefit from the old Dolby Digital / DTS soundtracks on DVDs as the extra space on Blu-Ray discs means more detail can be included in the soundtracks (a bit like comparing a lossless FLAC file with a 128kbs MP3 music file). You can also get uncompressed PCM soundtracks which, as they say, are uncompressed and thus should sound even better.