When used as an adverb it means either "somewhat" or "very" depending on intonation.
"The music there is pretty loud," with the verbal emphasis on "loud," means that the music is very loud. But if the emphasis is on "pretty" with a softer "loud," that expresses doubt, as if the music is loud but not as loud as might be expected.
* "I thought that the word "pretty" is only used to mean "beautiful," but recently I discovered that it has many different meanings."