In"The Glass Menagerie," what is the meaning of the lights going out in the middle of dinner and candles being lit to replace electricity?

The lack of electricity is an indication of Tom's irresponsibilty in not paying the bill.  However, the darkness and candlelight afford Tennessee Williams better opportunity for the expressionism of his play, "The Glass Menagerie," thus furthering the theme of illusion.  In such dim light, Laura can overcome her shyness surrounded by shadows.  The expressionistic stage direction states,

Jim lights a cigarette and leans indolently back on his elbos smiling at LAURA with a warmth and charm which lights her inwardly with altar candles.

The "altar candles" represent the unearthly qualities of Laura and of her lack of connection to the real world.  Truly, she is like her little glass animals who merely reflect the light.  At the close of Tom final speech, Laura is seen blowing out these candles, thus ending the play.  Tom says,

'For nowadays the world is lit by lightning!  Blow out your candles, Laura--and so goodbye...'