A fallen woman is a term used to describe a woman who is seen as having done something bad or wrong, usually related to sex. This term is old-fashioned and isn't used very often these days because it's not very polite to judge people based on their actions.
Sometimes people in the past thought that it was bad for women to have sex before they were married or to have sex with someone who wasn't their husband. They believed that women who did this were not good or respectable people, and they called them "fallen women."
However, we now know that it's not fair to judge people based on their sexual behavior. Everyone has the right to make their own choices about when and with whom they have sex. So while the term "fallen woman" used to be used a lot, we don't use it much anymore because we know that it's not kind or fair to label someone in that way.