What is the musical WICKED about:)xxxxx?

Im 13 and go to a drama class. We Leart the song defying gravity and they want people to try out parts, my teacher said(in front of everyone) i should try for the main rule of elphana and my friend to try for glinida, but she's to embarrassed and whats just a background part. so what's it about and any tips thank you xoxoxox