“I was pleasantly surprised.”
You bought a cheap bottle of wine, but you actually liked it. You say this when telling your friend about this wine.
I was pleasantly surprised.
You can be "surprised" in both positive and negative ways. When you are surprised in a positive way, you can say that you were "pleasantly surprised":
A: How was the art show?
B: You know, I thought it would be boring, but I was pleasantly surprised.
The word "pleasant" means "good". So why do people say "pleasantly surprised" instead of "positively surprised" or "surprised in a good way"? There's no real reason, except that these words usually go together. This is an example of collocation.