I hope I'm not going to spoil this for you, but here goes. Elphaba's and Nessarose's mother was not faithful to her husband, the mayor (or gov.) of Munchkinland. She had an affair, and the man she had an affair with is the one singing "Have another drink, my dark eyed beauty..." in a "re-cap," if you will, of Elphaba's birth/early childhood. The book goes into this much, much further than the play. Anyway, you don't find out until the end of the show that the man the Witches' mother had an affair with, ends up becoming the Wonderful Wizard of Oz. I hope this clears some stuff up for you, w/o giving too, too much away!
-Vincent