Defo, you know PRETTY as an adjective, when we say that a girl is PRETTY.
Yet, PRETTY is also used as an adverb, and you’ve heard that too.
It means QUITE and used a lot in informal conversations.
Mind, that as it’s an adverb, it can be used like this:
The joke is PRETTY FUNNY.
I’m PRETTY BUSY.
He runs PRETTY quickly.
Your nephew reads PRETTY WELL.
It’s a PRETTY INTERESTING MOVIE.