Handmaid is an old fashioned word for a housekeeper or servant. Wealthy people living in grand houses once employed handmaids to clean for and serve them.
You're most likely to come across the word handmaid in a book these days, since in-house "help" is more likely to be called a "cleaner," "housekeeper," or, rarely, a "maid." Years ago, handmaids were employed to work closely with their employers, sometimes as "ladies-in-waiting," assistants to queens and other royals. The term comes from maid, "young woman," and the "close-at-hand" sense of hand.