So many new Enforcers think that all there is to the profession is doing damage and being able to take damage, but have little consideration for their role in a team. This will explain to you not only what aggro is, but how you, as an Enforcer, need to manage it.
Aggro is a term used to define the attention of a mob. So, when you "have aggro," a mob is attacking you. In AO, there is a "hate list" for a mob and the player doing the most damage (or posing the highest threat like when a Doctor is healing players and keeping them alive so they can continue to do damage to the mob) will cause the mob to switch aggro (or attention) to that player.
In a team, an Enforcer's main job is similar to a healer in that you help keep the rest of the team alive. Where a healer heals, you take damage so the rest of the team doesn't in the first place. This also makes the healer's job easier because they can concentrate on fewer team members to heal. An Enforcer and a Doctor in a team are a very great asset.
There are several tools that help with getting aggro and maintaining (or keeping) it. The most well known is the Mongo nano line. This is like an area taunt in that it will attract the attention of all mobs within a 1m radius. There are also single taunt lines that I haven't actually played with but have been meaning to try. These will effect only the mob that you have selected and not have the radius effect like Mongo. Check out Auno.org for all Enforcer nano programs.
We Enforcers don't have very large nano pools so that is where devices like Da Taunter! (for Atroxes only) and Aggression Enhancers (all breeds) come in handy. These devices do not do a good job of getting aggro, but work great for helping to keep it. Place the device in your Utils 3 slot in the Wear panel then create a shortcut to it on your hot key bar where you keep all of your regular combat stuff. Once you enter combat and have aggro, hit the hot key for it and keep doing this during the battle to help maintain aggro. The recharge is pretty fast (like 5 or 6 seconds), so you'll be doing this a lot. It's not a guarantee to maintain aggro, but it really does help.
Using Mongo depends on your surrounding. Never use Mongo outdoors at a place where there are a lot of mobs as they will all attack you (trust me on that :p). You also need to be careful in missions. When I am tanking during battles, I move my camera around a lot because I am always looking for open doors with other mobs in view because I don't want to draw them out by using Mongo. Often times, it can be a tough judgement call - do you let one of your team members die so as not to draw in mobs from two adjacent rooms or do you try to move out of line of sight of the other mobs and take a chance? Your decision might save the other player or risk the entire team - particularly if you don't have a good calmer with you.
One side benefit of Mongo is that is also runs a pretty nice HoT (Heal Over Time) for a short period of time. This can be a real life saver. It's also handy while soloing as Enforcers have big health pools which can take a while to fill. By timing when you use Mongo (and being careful of your surroundings), you can keep your health up fairly well. I even use it in between fights when I'm soloing missions just to speed up the healing process.
While I'm talking about Mongo, you should also be aware that the Challenger nano line locks your nano skills for 57 seconds so you won't be able to use Mongo for that period of time. During battles, I don't use Challenger right away and will generally wait until the mob is down to 60% or so health just incase there is a problem and I need to Mongo or Mongo again. Additionally, by waiting a little bit to use Challenger, the nano usually lasts until you get to the next mob and will still be running when you attack giving you a little extra use out of it.