Walter had grown up in Mama's house with Mama making the decisions for the family. Walter has been a bitter black man, trying to make it in a white man's world. He feels that life has passed him by. He makes excuses for his bitterness, claiming that others have better advantages in life.
Walter drinks and practically gives away his father's insurance money. He then decides to give Mr. Lindner what he wants by selling out. He decides that the family will sell the house and not move into the new neighborhood.
When Mama asks Walter to explain to his son Travis why he is making such a decision to sell the family's new house, Walter thinks about his decison. When Mr. Lindner comes over, Walter has a change of heart. He explains to Mr. Lindner that the family will indeed move into the new house. That is when Mama exclaims that Walter has finally become a man. He finally makes the right decision for the family. Walter could not sell the house with Travis looking up to him. Walter comes into his manhood as he looks first at Travis and then Mr. Lindner. He makes the right decision based on what is best for the family. For once, he is thinking about others and not himself.
At the end of A Raisin in the Sun, Mama says this of Walter as she is speaking with Ruth and is just about to leave their old apartment for their new house. She is referring to Walter's maturation, as Walter originally proposed spending the insurance money from his father on a liquor store, a cynical idea fueled by dreams of material wealth.
In the end, however, Walter refuses to accept the money from Lindner, a white man who wants to pay off the Younger family so that they won't move into a white neighborhood. Instead, Walter tells Lindner, "And we have decided to move into our house because my father--my father--he earned it brick by brick." Walter recognizes the value of his father's work and the necessity of carrying on his father's work by moving to a house where his family can have a better life. Mama compares Walter to a rainbow after the rain because Walter's journey towards this realization about what to do with the insurance money has been stormy. Walter has fought with everyone in the family, including Mama, his wife Ruth, and his sister Beneatha, but in the end, he emerges as a proud and wise man.