In informal, conversational English, you can use this as an adverb. It modifies, or gives more information, about the adjective.
e.g., This is good. This is pretty good.
When you use it as an adverb, think of it as meaning approximately 65-75%.
See the STORY Confessions of a Whiner example: You know, Laura, it’s pretty cold out here with that wind.
This adverb can be a little difficult to learn, because the “amount” changes a little depending on how quickly you say it.
If you quickly say, “That movie was pretty good, wasn’t it?”, you mean that you enjoyed the movie and would probably recommend it. (65-75%)
If you say pretty slowly, “Yeah, it was pretty good,” you now mean that you didn’t really enjoy the move and you probably wouldn’t recommend it. (You feel it was about 40% good.)