One of the main themes of the play is the power of illusion. In The Glass Menagerie, nothing is ever quite what it seems, with illusion taking on a reality all of its own. This is the inevitable result of action that's constructed from the mind, pieced together from the shattered fragments of memory. One's memories can often play tricks, and the unreliability of memory admirably serves Williams's resolutely anti-realist approach.
Memories, of their very nature, tend to be selective, and we're left in no doubt that Tom's highly selective memories present us with a version of events steeped in illusion. The illusionary atmosphere of the play is heightened by the use of music and expressionistic lighting techniques, which deliberately prevent anything like a realistic reconstruction of events from emerging.
Laura's collection of glass animals—the glass menagerie of the title—symbolizes the general sense of illusion on stage. The achingly vulnerable Laura is simply incapable of living in the real world, with all its harshness and cruelty. So she's forced to retreat into a fantasy world, a world where she is guaranteed to find safety and repose.
For her part, Laura's mother Amanda also prefers illusion to reality. She seems unable to accept Laura's fraught emotional condition and persists in her unrealistic plans to have her daughter married off to some eligible bachelor. To a large extent, this is a result of Amanda's remaining stuck in the past, a much happier time when all those handsome gentlemen callers came beating a path to her door. For Amanda, no less than her daughter, the big old world outside is just too hard to deal with, and so living out an illusion acts as a kind of security blanket.