Femme is a slang term for a lesbian woman who dresses and acts in a way that society considers typically feminine. The opposite of being femme is being butch.
There are a lot of assumptions about what it means to be femme, for example, that femme women are very "girly," have long hair, and are only attracted to extremely masculine women. However, like most stereotypes this doesn't fully capture all femme women's experiences. For example, plenty of femme women have short hair and enjoy sports.
Some women feel comfortable adopting the femme label because to helps clarify their identity. Others find labels like this limiting.
Choosing to identify as femme, butch or anything else is a personal choice and one that you may or may not feel accurately captures who you are.