As I understand it, they are people who can manipulate elements: not the scientific sort of elements, but things like water, fire, earth, and air, by moving their arms and legs. I had a waterbender on my crew, before he vanished: he showed me, a few times.

[At the mention of the waterbender in question he droops a little--it has been a while since he has heard anything of Noatak. It is very annoying, how frequently his crew disappears these days.]