Amanda, Tom and Laura's mother is both a symbol and an image in the play. Symbolically, she represents a time gone by, a genteel time, when a lady was called upon by her gentleman callers and her future was secured through a prosperous marriage.
Laura, on the other hand has no such environment on which to rely for her future. Amanda, between her behaviors, attitudes and her southern charm, keeps the old south alive in the play, so that the reader is constantly imagining what her life was like. On the other hand, Amanda's image, of a washed out old debutante who is stuck in the past reflects the hopelessness of the situation in the family.
Tom keeps the image of his missing father alive through his attitude toward Amanda. Tom is a symbolic stand in for his long gone father. Amanda, in the end, chases Tom away too.
Laura, like Amanda will end up alone. After Laura has her romantic interlude with her old high school friend, Jim, and he leaves abruptly because he is getting married. It is easy to imagine that Laura will never leave the house again.
The dreary apartment reflects the melancholy of the lives of the characters. The darkness that Tom leaves his mother and sister in is both figurative and literal. They are left in physical darkness as they face the darkness of their uncertain future. A future that at the end of the play consists of no income, no job and no prospects.