Various definitions of the symbolism of breaking the glass which can be explained to the guests at the ceremony :

• This symbolizes the breaking down of barriers between people of different cultures and faiths. After the glass is broken everyone yells "Mazel Tov," which means good luck. 
• The fragility of the glass suggests the frailty of human relationships. The glass is broken to protect this marriage with the implied prayer: "As this glass shatters, so may your marriage never break."
• Shattered glass symbolizes the fragility of our relationship and reminds us that we must treat our relationship with special care. This custom was also incorporated into the ceremony to remind everyone that even at the height of personal joy, we must, nevertheless, remember the destruction of the Temple in Jerusalem. On a lighter note, thought by some to be the last time the groom ever gets to put his foot down. 
• It teaches us that in times of joy we must also realize life brings sadness and sorrow. The sound of the breaking glass is said to frighten away evil spirits who might spoil this joyous occasion with their mischief. It also warns us that love, like glass, is fragile and must be protected. The promises made by the bride and groom, like the broken glass, are irrevocable. The breaking of the glass also serves as a reminder of the destruction of the Second Temple in Jerusalem in 70 CE and all subsequent sufferings of the Jewish people. Even in a moment of such great joy, we are asked to remember that there is still pain and suffering in the world and that we have a responsibility to help relieve some of that suffering.
• It reminds us that love, like glass, is fragile and must be protected. The glass is broken to protect the marriage with an implied prayer: "May your marriage last as long as it would take to repair this glass." The shattering of the glass concludes the ceremony on a high note. 
• At the conclusion of the ceremony, we are breaking the glass together--as a reminder that working through the challenges and celebrating the successes of life is best done together. One interpretation of the custom is that even in the happiest times, we must remember there is still much suffering in the world. For the bride and the groom, the shattering of this glass also symbolizes the breaking down of barriers between people of different cultures and faiths.