A Raisin in the Sun is the work for which Lorraine Hansberry is most well known. The play debuted in 1959, and the movie, starring Sidney Poitier, appeared in 1961.
Before answering your specific question, I'll summarize the plot up to the point with which we need to concern ourselves.
The story centers on a poor, urban black family who comes into some life insurance money when the head of the household dies. The money, $10,000, creates conflicts within the family, as Walter, now the oldest male in the family, wants to use the money to invest in a liquor store start-up with two other men. Walter's mother, Lena Younger (wife of the deceased man, referred to hereafter as Mama), wants some of the money to go toward Beneatha's (her daughter and Walter's sister) medical education. As the widow, the money belongs to Mama. However, she does not approve of Walter's plan to invest in the liquor business. After using $3500 as a down payment on a new house, she gives him the rest of the money, telling him to hold out what Beneatha needs for college and put the rest in a checking account under his name. It's up to him to manage the checking account as he sees fit.
At this point, Walter's spirits improve significantly. Shortly afterwards, a white man arrives at their apartment and offers to pay the family NOT to move into the white neighborhood. The family is insulted and sends him away. What the audience doesn't know yet is that Walter has taken the entire 6500 that Mama gave him and given it to a man named Wily, who is supposedly one of the liquor store investors. As the family is in the process of preparing to move, Walter learns that Wily has cheated him and run off with the money.
The family is devastated. Obviously they are particularly angry at Walter, who is suffering more than anyone else over the situation. Walter leaves the apartment, but the family doesn't know where he goes. When he returns he tells them that he has called the white man from the neighborhood and told him they will take his money.
Okay—now this is the key part of the story. Walter is about to change. At this point he is feeling victimized and is willing to shame himself and his family by being paid off to stay out of the white neighborhood. The rest of the family feels that this is even worse than the theft of the $6500. When Mama asks him how he will feel when he does this he goes into a long, dramatic monologue in which he pretends to speak to the white man:
And maybe—maybe I'll just get down on my black knees . . . (He does so, Ruth, Bennie and Mama watch him in frozen horror.) "Captain, Mistuh, — Bossman (groveling and grinning and wringing his hands in profoundly anguished imitation of the slow-witted movie stereotype.) A-hee-hee-hee! Oh, yassuh boss! Yasssssuh! Great white — (Voice breaking, he forces himself to go on.) — Father just gi' ussen de money fo' God's sake and we's—we's ain't gwine come out deh and dirty up yo' white folks neighborhood ..." (He breaks down completely.) And I'll feel fine! Fine! FINE! (He gets up and goes into the bedroom.)
At this point the rest of the family sits amid the packing crates feeling terribly low. Beneatha says that Walter is no brother of hers, and Mama chastises her for failing to feel sympathy for what he has been through. At this point Walter seems to have lost everything—his dream of owning his own business, the family's money, and now, at last, his self respect and the respect of his own family.
But it turns out that Walter's tirade has had a therapeutic effect. The audience doesn't know it yet, but as he sits silently his outlook begins to change. When the white man arrives to pay them off, Walter has changed his mind. With his young son Walter Jr. at his side, he tells the man that they are not going to take the money, that they are a proud people and that they will try to be good neighbors, but that they are going to move into the house.
Walter's change appears to be a result of hitting the bottom and then getting back up again. When he heard the words coming out of his mouth, it gave him a chance to really look at himself. He realized that after losing so much, the only thing he had left that mattered was his family and his dignity, so he responded with an act that satisfied both.