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