It means "half".
I've been speaking English for about forty years now, and I've studied it, and I still study it, and I read a lot, and yet I still get surprised by random words that seem -- for all appearances -- to be interlopers into the language, yet aren't.
Moiety is one of those. It looks new and strange to me, but it's been around since the early 15th century, and it's not even listed as "obsolete" or "rare". And still, it surprised me the other day.
Thanks, English. Thank you for being so astonishing.