According to MyJewishLearning.com, stomping on a glass is one of the best-known features of Jewish weddings. Traditionally, the groom did the deed; today the couple often share the honor/pleasure, smashing one or two napkin-wrapped glasses.
Few Jewish symbols have a single explanation, and this one is downright kaleidoscopic. The custom dates back to the writing of the Talmud :
Mar bar Rabina made a marriage feast for his son. He observed that the rabbis present were very gay. So he seized an expensive goblet worth 400 zuzim and broke it before them. Thus he made them sober. (Berakhot 5:2 )
In other words, where there is rejoicing, there should be trembling.
The fragility of glass suggests the frailty of human relationships. Since even the strongest love is subject to disintegration, the glass is broken as a kind of incantation: “As this glass shatters, so may our marriage never break.”
The crash of glass ends the hush of mythic time under the huppah, and the world rushes in. Everyone exhales, claps and shouts, “Mazel tov!” The celebration begins.