"Aggro" or "threat" describes when you get the attention of enemies around you and they start attacking you.
A tank's role in any team is to maintain aggro so that the enemies only attack the tank and not any of his team members. If the tank can do this then the other players who are the damage dealers can focus on taking down those enemies that are ignoring them as they attack the tank. At the same time the healers role will be to make sure the tank doesn't die so that the damage dealers can do their thing.
Tanks maintain aggro by using a Tanking stance, for example, the tanking stance for a Trooper Vanguard would be the Ion Cell. Then I'd use moves that the Vanguard has to maintain that aggro so that the enemies attention is always on me. If one of the damage dealers happens to pull aggro away from me, I have a couple of abilities called "taunts" which I can use to reacquire aggro so that that player is no longer being attacked.
I haven't played a Jedi Guardian tank so I don't really know how hard it is for them to maintain aggro, but I'm finding that the Vanguard does a pretty awesome job.