Agro represents the hate a NPC has for a PC. It is used in many ways, such as to describe the fact that a player is on a mob's hate list ( I can't regen, I've aggroed the mob!) or to describe the action of doing something to become hated by a NPC ( Go aggro that mob and bring it here). Agro can also be used to describe that a mob is KOS ( Watch out, that dragon is aggro!) or even that a mob is currently engaged by another player ( That other group aggroed the named before us!).