I answer with the authority of being a native German speaker and having graduated in philosophy ;)
Back-world is a bad translation here. Presumably, the translator has mistaken the term "Hinterweltler" as being a misspelling and semantically identical to the word "Hinterwäldler", which means backwoodsmen or hillbillies. The German original "Hinterwelt" literally means "behind-world" or "beyond-world", where a valid translation would be otherworld or, more philosophically, the sphere of the transcendental.
In the context of the chapter, it becomes clear that he is writing about those who adhere to and propagate the belief that there is some transcendental world beyond human life and body, a world of gods, souls, and spirits. He starts the chapter with the remark that Zarathustra himself once committed to this erroneous belief ("Gods work"). Thus, the term "Hinterweltler", here translated as "backworldsmen", is simply a neologism referring to people believing in a transcendental reality.