The word pretty has two main meanings that work quite differently:

1. When describing appearance, pretty means pleasing to look at in a delicate or graceful way. A pretty garden might have colorful flowers arranged in patterns. A pretty sunset might show soft pinks and purples across the sky. People often use pretty to describe things that are attractive without being dramatic or overwhelming. You might call a dress pretty, or a melody pretty, or even handwriting pretty if it looks neat and pleasant. The word suggests something appealing and nicely put together.
2. When modifying other words, pretty means “fairly” or “somewhat.” If you say a math problem is pretty difficult, you mean it's quite challenging but not impossible. If your day was pretty good, it was better than just okay but maybe not fantastic. In this sense, pretty softens whatever word follows it. “That's pretty much everything” means “that's almost everything.” “I'm pretty sure” means “I'm fairly confident but not absolutely certain.” This usage helps express the in-between feelings we have about things, when words like “very” would be too strong and “slightly” would be too weak.