Hi! I edited your question and moved it to the proper topic, Death of a Salesman, as you had it under the title Death of a Traveling Salesman. This latter title refers to a short story by Eudora Welty, whereas the characters of Biff, Happy, Willy, etc. belong to the play by Arthur Miller Death of a Salesman. The titles are quite similar, so is completely understandable to get a bit confused :) .
In act 2 of the play a lot of things take place. In this act you see the Loman clan's highest "highs" of hopes, as well as their lowest "lows" of deception. There is also evidence of hope in the future plans when Biff and Happy.
Yet, all of this comes crashing down when Biff makes a huge realization that nobody else in the Loman family has made as of yet: That Willy Loman's view of the world, of success, and of the so-called American Dream is so abstract that it has led nowhere, but dreaming.
The Lomans have all allowed Willy's notions to feed their heads. Nothing concrete ever really happened to move the family forward, therefore, the family has gone through life doing nothing but dreaming on a better tomorrow that will never come-- because Willy continuously looks for loopholes and shortcuts- such as his career- to "make it big" unsuccessfully.
Back to Biff-
Before coming to his realization, Biff was brought up being told that he was made to be successful, and above everyone, since his looks made him to be "well-liked" and victorious. All of those were parts of the dictum imparted by father Willy. Life's loopholes, such as looks and popularity, were things that Willy would tell Biff that really matter. Logically, Biff grew up thinking alike.
Then, reality set in. Biff's flunking Math caused that he could not get the credits to graduate high school and go to college (as was the plan). That, paired up with the discovery that his father, the self-proclaimed "man of New England", was having a lowly affair with a woman on the road, totally deflated Biff's ego and of his view of his role in the world. Biff was nothing but the hot air his father blew into him. Everything was a big charade.
All is summarized in the words he finally dared say to Willy:
BIFF: I am not a leader of men, Willy, and neither are you. You
were never anything but a hard-working drummer who landedin the ash can like all the rest of them! I’m one dollar an hour,
Willy I tried seven states and couldn’t raise it. A buck an hour!
Do you gather my meaning? I’m not bringing home any prizes
any more, and you’re going to stop waiting for me to bring
He was no "Adonis" as his father would have said. He, and all the Lomans, are nothing but silly dreamers that have accomplished none of the things they talk about so much. This was a huge blow for Biff, and this is why he conveys these additional words to his father
BIFF(crying,broken):Will you let me go, for Christ’s sake? Will you
take that phony dream and burn it before something happens?
The significance of this message is not just bringing reality for once into the dynamics of the Loman family. It is the recognition that the dream, the idealization of someone or something is non productive. That this "American Dream" is entirely in the eyes of the beholder of the dream. To Willy, it meant to be well-liked and successful. This translated into Biff making other idealizations in his head that had nothing to do with him. Biff wanted out of the fantasy, and he wanted the others to do the same. Forget the dream. Go with what you actually have. Stop the fantasy...bury it before it keeps hurting people.
Biff was not "a big deal". He was happy, or content, with being outdoors and working the land. If this was a problem for Willy, then which is the dream, then? The one others have for you or the one that you plan for yourself? That is another huge meaning within Biff's words too.