Aggro (aka hate) determines whether a mob will attack you. In RF a monster can attack one player at a time, and when you "pull aggro" on the mob, it attacks you. A red "bull" symbol shows above the mob's head when you pull aggro.
You can get aggro by doing any of the following:
- Doing the first hit on a mob
- Doing a lot of damage (dps) to the mob
- Debuffing the mob
- Healing another player that is attacking the mob
- Purging another player that is attacking a mob
- Running supporter buff for someone who is attacking the mob
Tank classes (like Black Knight, Mercenary, and Shield Miller) have class skills that force the mob to attack them, pulling aggro off of others.