There are several ways of looking at the candles. First, they establish a more muted tone - you might even call it romantic. That less harsh light perhaps allows Laura to be more open with Jim; it gives her courage. That might lead to an interpretation that the candles symbolize hope. Hope that Laura might be able to connect with Jim, hope that Amanda's dream of a Gentleman Caller providing a future for Laura, hope that if Laura is taken care of, Tom will be able to strike out on his own and leave the shoe warehouse, etc.
It makes Tom's final line more heart-breaking - "blow out your candles, Laura." There is ultimately no hope for this socially and physically (at least in her mind) handicapped young woman. There is no place in our society for such a misfit.
Tennessee Williams gives Laura a physical disability that is at times hardly noticeable and at other times very prominent. This is similar to Laura's emotional and mental state. Laura looks perfectly normal and functions inside her family, but is unable to successfully hold a job outside the home. She is filled with devastating anxiety that causes her physical incapacity. Such as when she throws up when she is given a typing test at the business college. She appears normal, but on closer examination, she is not.
Although the limp can be hidden, like her emotional instability it cannot be ignored completely. The limp reminds us that Laura is fragile and damaged, both physically and emotionally.
When Laura blows out the candles it is symbolic of the darkness that now descends into her life and Amanda's. Tom has left them literally in the dark by not paying the electric bill. And we don't know what happens to Laura and Amanda with no money and no income. So the darkness is both figurative and literal.
Laura's limp symbolizes her lack of self-esteem and her insecurities about being independent (as far as being able to function in the "real world"). It can also represent her feeling of inferiority since she is considered physically "disabled." The candles that Laura blows out could represent the end of the life that she once knew, since Tom left town after the fiasco with his friend Jim and the dinner Amanda made for him. At that point, Tom realized that he could no longer stay in the house, even to take care of Laura. He had to move away from the house AND the town in order to purge himself of the life he once lived. However, this didn't keep him from thinking of Laura, who he loved very much.
The limp is a convention used to set Laura apart from "normal" girls (in high school) and other young women (as an adult). It also reinforces Amanda's refusal to acknowledge, at least to others, the reality of their situation: "I've told you to never, never, use that word", and instead of doing so, Amanda throws the condition back onto her daughter by declaring that, in Amanda's youth, girls minimized problems such as Laura's limp by focusing on their more desirable qualities.
The candles symbolize Tom's memories of Laura. While he often spars with Amanda, he does love and care about her in his own way; this is even more true of his sister. Yet, as the description of Tom's character in the play's "dramatis personae" indicates, Tom must act "remorselessly" in order to "escape from a trap." But he feels keenly the rift, foreshadowed by the power cut during the dinner with the gentleman caller, that has been forcibly created by his escape, and as the years pass, he is forced to admit to himself that he will be unable to forget his love for her. Finally Laura herself blows out the candles, hinting that she has been more successful than Tom at dealing with their split, or perhaps, even, that she has died.