In the Roman Catholic Church, the Requiem Mass is the funeral mass for a deceased person. The word "requiem" may also refer specifically to a solemn prayer or piece of music offered in hopes of bringing the deceased peace in their passing.
In the case of Death of a Salesman, the requiem serves as a kind of post script, the means of bringing closure to the lives of the characters involved. Willy, the deceased, could certainly use prayers for peace and tranquility, given the life he lived and the manner of his death. His sons struggle to find peace but are left with questions: for Biff, the continuing struggle to find himself in the wake of Willy's unsupportive parenting; for Hap, the hope that he would succeed where Willy failed; the fear that he would ultimately also be a failure.
I'm gonna show you and everybody else that Willy Loman did not die in vain. He had a good dream. It's the only dream you can have—to come out number-one man. He fought it out here, and this is where I'm gonna win it for him.
Linda is left to deal with the suicide of her husband and the ironic freedom achieved from her abusive husband and from the financial burden of the house that they had shared.
I made the last payment on the house today. Today, dear. And there’ll be nobody home. We’re free and clear. We’re free. We’re free. We’re free.
The Requiem offers prayers for peace for the souls of all the Loman family members; they all are in need of those prayers.