They are gender neutral pronouns which are intended to replace he/him or she/her pronouns while leaving all other sentence grammar intact. Compared to They/them pronouns which function differently grammatically. They're also singular in nature. Which they/them can be used singularly, but Ze/Zir, Xe/Xem/Xyr, Vi/Vis, etc. are always singular, just like he/him or she/her. It's very easy to swap them out because English doesn't have much gramatical gender in it, unlike other languages like spanish. Although I hear there are strides made to create gender neutral grammar in spanish.