Why do you think the prioress wears this brooch? What does she want people to think about her wearing it?

A broach would have been a pin of some kind hanging from the edge of her rosary. The broach was probably made of gold and the "A", a Latin inscription that roughly translates to "love conquers all." I think she wears this to make people think she is a pious woman. She tries hard to make it seen.