An undine (or ondine) is a water nymph in some European mythology. From what I've read, the name is commonly used to reference one nympth in particular who fell in love with a mortal, lost her immortality, and was ultimately betrayed. I don't know much about the myths beyond that. In any case, I assume the play has something to do with her story or with a story with symbolic references to hers...? I'm afraid I've never read/heard of it.