[anbende]

I thought it was referring to the planet being "deep" in the sense of being in one of the lower zones of thought. And the Fire was the conflict of galaxy-spanning import that was playing out there.

But maybe there was more to it?

By contrast, the meaning of the title of the sequel "a deepness in the sky" is fairly literal once you get to know more about the alien species and their habits.

[Vecr]

"A deepness in the sky" is literally (well not quite, but it's the text in the book) a line by one of the characters.