What does Aggro mean?
an MMORPG when you "piss off" an NPC(non-player character) and it enters a combat state with your character. Most often used when multiple NPC's are attacking you because you got too close or you attacked them.
"I just aggro'd 15 level 80's! I need a heal!"
That's basically it but it is more important than that, consider having two players and a bunch of NPCs, one player has a weak defence and low health but really strong attack, then the second player has a very tough defence and high health but barely any attack, without using aggro the weak player would get killed of quick and the tough player would last longer but soon die being unable to kill off the enemies due to his low attack.
If the tough player has more aggro and the enemies think he is more threatening, then they will attack him. This means the weaker player with higher attack can deal greater damage whilst the tough player takes the damage for him.
You can also throw a healing based unit into the mix. This unit is weak but can heal other units a great amount, so can keep the tough player alive in a long battle.
Edit: The aggro system could help implement stealth units too, maybe using a negative aggro value or something?