Well, for Willy, his first-born son, Biff, is everything in the world. Even back when Biff was a football star in high school, Willy wasn't doing too well any longer as a salesman. This fact made him tired and depressed, but at least there was Biff. Biff was someone who Willy could believe in and admire when everything else was disappointment. And so Willy used Biff as the repository of all of his hopes and dreams... and he pumped his son full to the brim with his expectations and praise.
How did Biff take to all this adulation? It seems he loved it and believed it. One might assume, though, as he matured, Biff could have thought his father went a bit over board with his praise of him, and he might have begun to suspect that his father lived through him. How many times have we seen the parents of high school athletes who are more competitive at school sporting events than their own kids? Why do you suppose that is? What do their kids think of it?
With this symbiotic relationship as the background, Biff's trip to Boston becomes even more horrific. Biff has flunked math, and he goes to Boston, where his father is supposedly working, to get Willy to come home and persuade the teacher to change his grade. There, in the Boston hotel, Biff finds his father with a woman. It is clear to Biff what is going on, and he breaks down right then and there. All the years and years of hot air and hype disappear, and Biff is left with nothing but tears and bitter disappontment in his father... and in himself.
As Death of a Salesman begins, Biff has returned home after years of trying to find himself, the real self away from his father's hopes and dreams for him, away from his father's attempt to live through him.
In the end, Biff comes to terms with who he is, and Willy commits suicide... still dreaming