It's the natural energy found in lightning, and in the sparks produced sometimes from rubbing against a coarse fabric or such. It's commonly used here and in some other worlds to provide power for mechanical devices like these to function.

*his world relies heavily on magic, itself, but there's still enough understanding of the science behind spells' effects that he has a good idea how to explain*

I mean that those of us who've found ourselves suddenly here with no explanation apparently invariably are drawn from worlds other than this one. I'm from the Roland Empire in the world of Menoris, myself.

*names he definitely doesn't expect her to recognize*