“Hygiene” in the context of macro systems refers to the user’s code and the macro’s inserted code being unable to capture each other’s variables (either at all or without explicit action on part of the macro author). If, say, you’re writing a macro and your generated code declares a variable called ‘x’ for its own purposes, you most probably don’t want that variable to interfere with a chunk of user’s code you received that uses an ‘x’ from an enclosing scope, even if naïvely the user’s ‘x’ is shadowed by the macro’s ‘x’ at the insertion point of the chunk.

It’s possible but tedious and error-prone to avoid this problem by hand by generating unique identifier names for all macro-defined runtime variables (this usually goes by the Lisp name GENSYM). But what you actually want, arguably, is an extended notion of lexical scope where it also applies to the macro’s text and macro user’s program as written instead of the macroexpanded output, so the macro’s and user’s variables can’t interfere with each other simply because they appear in completely different places of the program—again, as written, not as macroexpanded. That’s possible to implement, and many Scheme implementations do it for example, but it’s tricky. And it becomes less clear-cut what this even means when the macro is allowed to peer into the user’s code and change pieces inside.

(Sorry for the lack of examples; I don’t know enough to write one in Zig, and I’m not sure giving one in Scheme would be helpful.)