FLAC-HD / ALAC-HD are high resolution audio formats for audiophiles to enjoy more than three times the fidelity of a standard audio CD. Typically live shows in ALAC-HD and FLAC-HD are recorded at 24 bit / 48 KHz for maximum fidelity. For more information on 24 bit audio, please see this excellent FAQ.
FLAC-HD files cannot be played on iTunes without conversion but can also be played on other players like VLC Media Player
ALAC-HD files are native to iTunes but can also be played on other players like VLC Media Player.