The glass menagerie represents all of the Wingfield family, but especially Laura. It is precious, but fragile. It has to be protected from life, or else it will shatter. It is unnatural, in that objects that should be alive (animals) are made of other things (glass). It represents compensation for the real life she can't have, much as Amanda's memories of the past do. When Tom explodes in anger (at the end of scene 3), he shatters the illusion of peace in the family; as he heads out of the apartment, he accidentally breaks the menagerie, shattering it as well. (If you look at the stage directions there, you'll see that Laura "cries out as if hurt.") Tom tries to help clean up the fragments, but can't, much as no one can really reassemble a broken dream.