In The Glass Menagerie, the glass collection symbolizes aspects of Laura. On the one hand, she appears to be as fragile as the little animals. On the other hand, as an effect of light shining through them, the pieces give off beautiful colors that enhance their beauty.
When Tom inadvertently sweeps a part of the collection off the table with his sleeve, he feels that he has caused damage beyond repair. On a surface level of analysis, one might think that the broken pieces bear a negative connotation that will reflect on Laura.
However, if we seek a deeper symbolic meaning, we will find that the shattered glass is a way of forcing Laura to begin parting with the dreamworld in which she leaves. She is now one step closer to reality, having partly lost the possibility of taking refuge with her collection.
The symbol is completed when, later in the play, the unicorn loses its horn, another "accident" that befalls the only mythic piece. Through these two breakages, Williams suggests that now Laura is ready to face her circumstances as a young adult. Thus, all in all, what might seem a source of unhappiness takes on a positive hue and leads us to expect that Laura will mature and thrive now that she cannot turn to her "mirror image" -the glass menagerie.