One of the metaphorical deaths in the play is the death of the image Biff has of his father. He believes his father to be all powerful and infinitely strong. He travels to Boston to plead with his father to return to New York and get him out of the jam he is in. Upon discovering the woman in Willie's room, Biff immediately says that no one would listen to Willie. His image of his father is shattered and his personal dreams die.
See the theme "Appearance versus Reality".
Willy Loman is the central character in the play, but it is only Biff who sees the truth that Arthur Miller is trying to convey. Willy never does see the truth, which is that he is only a mediocre person with delusions of grandeur. Biff realizes that he has been injuring himself by trying to live up to Willy's unrealistic expectations. Biff tries to convey his new insight to his father but finds it impossible to communicate with him because Willy is just too old and too set in his ways to understand. Biff has a chance to start a new life because he is still young. It would seem that Happy, though still young himself, will not see the truth and will never change because he doesn't possess Biff's spirit or Biff's intelligence. Happy will go on trying to please his father even after Willy is dead. Willy must have been a good salesman to have sold himself so effectively to his wife and his two sons.