"Wonderful" means "great" or "very good". But it has a more extreme and more positive meaning. When you say that something is "wonderful", it means that it makes you really happy. Some examples include:
A: How was your trip?
B: It was wonderful. We had such a great time.
A: I got the job!
B: Oh, that's wonderful!
"Wonderful" is an ungradable adjective, which means that it sounds strange to use words like "very" or "a little" with it. That's because "very" is already part of the meaning of the word "wonderful". Some other ungradable adjectives include: