"You are a pretty girl."
Translation:Tu es une jolie fille.
"Jolie" is an adjective that refers to "beauty".
Certain adjectives are placed before the noun, some which you can memorize with the acronym "BAGS":
Beauty Age Good and bad Size (except for grand with people - see 3, below)
These descriptors - and a few others - are considered inherent qualities of the noun:
une jolie fille - pretty girl