Sorry this is a bit late, but the Netherworld is where the Rephaim reside. Think of it as "the spirit world" between the living world and the dead. You can read more about the different "planes" of The Bone Seasonhere on Samantha Shannon's blog. She discusses the different planes that exists. However, there isn't a whole lot of information, which may be withheld from us because it's discussed in future books.