It literally translates to "the young woman" but it's commonly used to refer to a young girl, typically a teenager or someone in their early twenties.
Here are some examples:
* "La muchacha es muy bonita" (The girl is very beautiful)
* "La muchacha está leyendo un libro" (The girl is reading a book)
* "Vi a la muchacha en el parque" (I saw the girl in the park)