What do Tom's final words of the play mean? What do the lit candles represent? Why does he say the world is "lit by lightning"?

In this passage, Tom is reflecting on his memory of his sister Laura as someone who will never be able to move in the world as others do because of her shyness and lack of confidence about her appearance. She is a "candle" in a world "lit by lightning," and this metaphor describes her quiet beauty and gentle ways, that may not be noticed or appreciated among people who are louder, more talkative, more assertive, or more glamorous. Tom fancies himself one of the kinds of people who Laura can never really fit in with: he is talkative, confident, and has a sense of adventure. He also has a deep urge to get away from his mother and his upbringing, and he knows Laura will never be able to get away as he has done, and this fills him with guilt. 

The lit candles are Laura's memory and the image of her Tom carries with him. He tries to distract himself with other things, but cannot forget her. He says "blow out your candles, Laura," and the stage directions of the play depict Laura blowing out the lit candles on a candelabra. This same candelabra was given to Laura's Gentleman Caller Jim by Laura's mother Amanda, in a scene that symbolizes what may be Laura's last chance to find happiness and a way out of her situation. The lit flame may symbolize Laura "carrying a torch" for Jim, who she has had a crush on since they were in school together. In a wider sense, it could also symbolize Laura's sense of hope for the future and her quiet way of doing things.

Tom says goodbye to Laura when she blows the candles out in this final scene. Is he finally extinguishing his memory of her? Does his goodbye mean he commits suicide? Does her act of blowing out the candles, which we assume happens on a nightly basis, mean she goes on as before while Tom moves further away from his old life? The play's ending is somewhat ambiguous on these points.