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