H ei s most likely using a parser, which is a separate software you can run while playing, to see everyones DPS against the mob, the mobs DPS, healing, etc. It's a great way to get info on the mob when learning how to win, to see who is pulling their weight, and to play around with groups, trying to maximize everyones spells, buffs, etc with one another. We use it in our raids and I love it.