"Like a prophecy?" Although she's skeptical of anyone really knowing the future, Zuko does believe in destiny. And Aang did have visions and things, so what Paarthurnax is talking about is... still difficult to wrap her mind around, but she thinks she has the gist of it.

The desperation they must have felt when they used it, though, that's a much more comprehensible thing.

"...I'm sorry."