An old maid is a woman who's not married, especially one who's very old. When you hear someone described as an old maid, you'll know it's a comment on the fact that she has no husband.
The term old maid is old fashioned and fairly derogatory — in other words, it's not nice to call someone an old maid. Back in the days when the idea of a woman remaining unmarried was a little bit shocking, most people had an old maid aunt or sister. And in the card game called old maid, which has been around since Victorian times, players try to avoid being left with the black queen, or the old maid.