Use the word pretty to describe something with a delicate and pleasant appearance. Something that’s pretty is less powerful and intimidating than something that’s beautiful, the way a pretty floral raincoat is less intimidating than a beautiful film star.
The adjective pretty describes things that look nice, but it can also be used jokingly to say that something’s bad. If you say, “That’s a pretty mess you got me into!” that means it’s a bad mess. Pretty has an adverb form that means something is present to a moderate degree, another way to say fairly. You might be pretty sure you’ll be done studying by 9:00. That means you’re fairly certain but not positive.