The absence of light in the cramped and dingy apartment where the Youngers live signify's a dreary future. This becomes especially clear when the darkness of the current apartment is juxtaposed to the hoped for light in the new house.
Until the family finally unifies and determines to move into the new house, the family's future is truly bleak. Mama, Walter and Beneatha all have troubles as a family unit. Walter and Ruth may not be able to weather another pregnancy and their relationship may not survive an abortion. The interpersonal future of this family is bleak indeed as the relationships unravel under pressure from a lack of opportunities, lack of space, and a lack of pride.
The darkness of their situation is symbolized by the darkness of the apartment and Ruth's specific hope for the family to remain intact is symbolized by her hope for light.
It signifies their "hope" of achieving the American Dream. It is the opportunity that they are so desperately grasping for. This goes hand-in-glove with Mama's plant. Mama says her plant doesn't ever get enough light or water. This is her "family" not getting enough opportunity to thrive. They need a "break". They need a "window of oppotunity", and they get it with the insurance money. It's a small window, because Walter has to "man-up" before he can lead his family the right way. Mama always wants her plant/family to have enough light and water, so they can "grow" together. Ruth, will eventually fill Mama's shoes, and she knows that having more "light"/opportunity, will be essential if the family is going to grow in a possitive direction.
This play, by the way, is a FANTASTIC piece to compare to Langston Hughes's poem, "Harlem". The Youngers answer the question that Hughes poses in his poem. What happens to a dream deferred? Yes...it turns ugly, so seize the opportunities you are given and don't set your dreams aside. (The message Beneatha is given by her boyfriend.)