Glauce is the princess of Corinth, whom Jason. marries despite the fact that he's already married to Medea. To Jason, Glauce represents a chance to finally live up to his royal heritage. By marrying her, he might actually get to be a king one day. In the end, though, marrying Glauce is the cause of his demise, when Medea takes a horrible revenge by killing Glauce, Creon, and Medea and Jason's own children.
For more on Glauce's tragic death check Shmoop's coverage of Medea by Euripides.