Light, in Tennessee Williams' play, "A Streetcar Named Desire," represents a few different things.
First, light, for Blanche, symbolizes her inability to face the truth in life. Blanche is aging. (Although she would like to make others, like Mitch, believe that she is younger than Stella, she is in fact older (about five years)). For Blanche, the light represents the truth behind her aging. In order to hide from the truth, Blanche uses colored paper to cover the lights in the room. These lanterns allow Blanche to hide from the truth by shading her physical appearance. Later, when confronted by Mitch, the lantern is ripped from the light so Mitch can really see her.
Second, light symbolizes the lack of reality in the play overall. Stanley's abuse of Stella, Balnche's rape, and the fantasy world both Stella and Blanche live in are all shown as not being as bad as it really is by keeping things hidden (represented by the covering of the light and lack of light in the flat).