Tom's monologue at the end of the play The Glass Menagerie serves as his conduit to provide some form of closure to the action, and to his thoughts. It also works as his last cry for help on behalf of his sister, Laura.

In his last words, he explains what he does after he leaves the house the evening of Jim's visit. 

TOM: I didn't go to the moon, I went much further -[...].

I left Saint Louis. I descended the step of this fire-escape for a last time and followed, from then on, in my father's footsteps, attempting to find in motion what was lost in space - I travelled around a great deal....

However, we find that Tom just cannot get over the guilt that he feels for having left the family, particularly, his crippled sister Laura. 

Then all at once my sister touches my shoulder. I turn around and look into her eyes ...

Oh, Laura, Laura, I tried to leave you behind me, but I am more faithful than I intended to be !

Tom utters these words because he is complete certain of the fact that Laura is totally unprepared for this world. Within the historical context of The Glass Menagerie (the late 1930s just before the onset of World War II) we find thatsociety is about to start moving at a very fast-pace; Laura simply cannot, and will never be able, to keep up with it. Moreover, the world is beginning to get rougher, more advanced, and more ruthless. Laura, and her mother, are nothing but antiquated pieces from a world that no longer will be. In Tom's own words:

 - for nowadays the world is lit by lightning ! Blow out your candles, Laura - and so good-bye.

Notice how the candle is a sign of the feeble and old-fashioned practices that still take place in the Wingfield home. These behaviors are in juxtaposition with what is taking place in the world outside the house. While the world outside of the house is lit by lightning, the only light that is seen in Laura's world is the subtle and weak flame of the candle. This is why Tom says 

I reach for a cigarette, I cross the street, I run into the movies or a bar, I buy a drink, I speak to the nearest stranger -anything that can blow your candles out !

Like the butterfly effect, Tom wishes that even the most minimal of his actions can somehow change fate,and help Laura catch up with life. He basically is saying that he, if he could, would do anything to help her get out of the retrograde world in which she lives, thus "blowing out" her candles, and moving her on to the real world; the world that has already found a way to light itself up, differently. 

Hence, these last words are an appeal that Tom makes to the world, to  destiny, and to Laura to please look forward and move on as best as she can.