Aggro is one of the Game Terms of the game RIFT. Aggro is jargon or slang for "aggravate" or "aggression". It is used most often in MMO terms to describe when someone has angered a mob to such an extent it is attacking them.
For example, if a mage throws a few too many fireballs around, he or she might be heard screaming, "I've got aggro!" Followed by "HELP, HELP IT'S ON ME!!!" Death follows seconds after that, usually.