In Arthur Miller's play Death of a Salesman, main character Willy Loman suffers a series of flashbacks throughout the play that represent both guilt and sadness over things that he fails to accomplish as a man, and as a father.
In Act 2, for example, Biff realizes the fantasy of his life and wants Willy to realize it too. He is the one who tells Willy basically to back off and realize what Willy really is. However, Willy refuses to accept his truth and wants Biff to do the same: If one stops playing the fantasy of their lives, the other would fall right after.
Biff: Pop! I'm a dime a dozen, and so are you!
Willie: I am not a dime a dozen! I am Willy Loman, and you are Biff Loman!
Similarly, we know that Willy continuously says how Biff is going to be great, and a "somebody." However, Willy speaks these things about Biff the adolescent football hero,and not to Biff the 34 year old Biff who is as oblivious of his fate as Willy is of his own.
During the restaurant scene, also in Act 2, Willy carries on with his fantasy of Biff by supporting Biff's idea of going to speak to Mr. Oliver so that he can ask him for a loan to start a huge new business. Despite the fact that Biff is actually a clerk at Oliver's company, he has allowed Willy to make himself believe that he is a partner in Oliver's firm. So in-depth is the enmeshment of these two men that Biff believes Willy's fantasies. In the end Biff is not even recognized by Mr. Oliver, he gets no loan, a huge humiliation and, to top it all, Biff steals Oliver's pen in a last act of desperation.
All this is what Biff attempts to disclose to Willy, while Willy continues to say that Biff and Happy are up to a "huge deal". It is Willy who continuously says to Biff that he is so well-liked that all will be alright for him, and it is Willy who tells Biff how much Mr. Oliver liked working with him. All fantasies, completely.
Therefore, in the final act when Willy commits suicide we realize that he still has some form of hope to extend his dreams through Biff. He wants to leave the boys with enough money to extend his legacy. Biff ends up choosing to find his dream elsewhere. Happy is the only one who still continues to follow Willy's "wrong dream."