Mazel tov is a Jewish exclamation of congratulations. "Mazel" means luck, and "tov" is the Hebrew word for good.
Anyone who's been to a Jewish wedding has witnessed the groom stomping on a glass wrapped in a napkin or cloth. In most cases, the groom breaks the glass after the rings are exchanged, stepping on it with his right foot. Then the guests yell "mazel tov!"
The origin of this tradition varies, but many believe it's supposed to remind everyone of the temple in Jerusalem's destruction and the commandment in Psalms 137:6 to "set Jerusalem above my highest joy" [source: Shulman]. A lighter interpretation is that it will be the first and last time the groom gets to put his foot down, as the bride will be the one to run the household [source: Ohr]!