What is the nature of Willy and Linda's relationship in Arthur Miller's Death of a Salesman, and how does it contribute to the events?

In Death of a Salesman, Willy and Linda Loman have a complex relationship in which Linda serves largely as an enabler to Willy’s dreams and fantasies.  A devoted and supporting wife – within certain limits – Linda takes care of the home and cooks the meals.  In addition to enduring Willy’s complaints and insults, Linda also serves as his defender against the criticisms of their sons, Biff and Happy.  From the outside, the Loman’s marriage would appear normal.  Behind the façade lies the truth about Willy’s affair and the dysfunctional relationships that permeate the home.  Further, it is eating away at Linda to see Willy, in the twilight of his life, not only failing to achieve any his professional ambitions, but to be suffering a deterioration of his mental capacity.  While she continues to humor Willy with respect to the latter’s dreams of a better life (“Before it's all over we're gonna get a little place out in the country, and I'll raise some vegetables, a couple of chickens . . .”), she knows that Willy’s gradual descent, both professionally and personally, has to be confronted, as in the following exchange when the increasingly delusional Willy is once again boasting of his importance to the company for which he works:

WILLY: They don’t need me in New York. I’m the New England man. I’m vital in New England.

LINDA: But you’re sixty years old. They can’t expect you to keep travelling every week.

Linda is trying to get Willy to slow down, recognizing that he can’t maintain his current pace as a traveling salesman driving long hours.  Willy lives in a state of perpetual denial, insisting that nothing is more important than to be liked and that, if only his bosses were supportive, he could be huge success.  Linda, however, knows better, and in one of the play’s most touching moments, in once again explaining and defending Willy to Biff and Happy, she laments:

“I don't say he's a great man. Willie Loman never made a lot of money. His name was never in the paper. He's not the finest character that ever lived. But he's a human being, and a terrible thing is happening to him. So attention must be paid. He's not to be allowed to fall in his grave like an old dog. Attention, attention must finally be paid to such a person.”

Willy treats Linda condescendingly, rejecting her comments with contempt:

LINDA: I’m just wondering if Oliver will remember him. You think he might? 

WILLY: Remember him? What’s the matter with you, you crazy?

Willy clearly takes Linda’s support for granted, and, symptomatic of the times, perhaps, devalues her opinion. 

Whether Linda is at least partly responsible for Willy’s plight is a little uncertain, but her rejection many years before of Willy and his brother Ben’s idea to go to Alaska (“Why didn’t I go to Alaska with my brother Ben that time! Ben! That man was a genius, . . . What a mistake! He begged me to go.”) lingers with Willy. His bitterness could certainly contribute to his disdainful approach to his wife.  To the extent that Linda blocked Willy’s best chance for wealth, then she could be considered responsible for his plight.  That, however, would be unfair.  Willy is delusional and a chronic liar.  Linda lived her life with Willy on a tightrope, needing to support him while trying desperately to keep him grounded.  She and Willy are both tragic figures, but Willy made his own decisions and lived the way he wanted.

What is the main point of Willy and Linda's relationship in Death of a Salesman?

Willy and Linda's marriage is troubling to examine. In many ways, it reflects the stereotypical 1950s marriage, with Willy having a public life outside the home and the responsibility for earning money and supporting the family financially. Meanwhile, Linda's place is in the home and her job is to shoulder the emotional support by raising the kids and taking on Willy's emotional burdens. She is also expected to be completely loyal to her husband, a role she fulfills admirably. 

In a way, Linda acts as a mother to Willy just as much as she does to Happy and Biff. When she throws their sons out of the house for upsetting Willy, the audience sees just how far she takes her responsibility to care for him. Ironically though, her focus on keeping Willy calm and not upsetting him helps to seal his doom: even though Linda is worried about his suicide attempts and hears her sons' anger and concerns, she fails to act for fear of upsetting Willy. She does nothing to prevent his eventual successful suicide and is in fact living in the same delusion that Willy has created for himself. This is most evident when she comments, surprised, at the low attendance of his funeral. Linda has been enabling Willy's delusion about his own success, because she has been living in it too.