Damage done is how much damage is done by a player (Including his pet, if this setting is enabled). This is normaly how you sort pure DPS players to see how much damage they are doing. This combined with the activity of a player can help you see which DPS players are slacking.
This shows how much damage players have taken, normaly the tank and off tanks will top this list. But if some DPS, support and healer is showing high numbers it could be a sign that he is attracting to much aggro.