Mazel tov is a special phrase that Jewish people use when something good happens to someone, like when they get married or have a baby. It is like saying "congratulations!" in English.
The phrase comes from Hebrew, and it actually means "good luck" or "good fortune." When someone says mazel tov, they are wishing the person good luck and happiness for their special occasion.
Usually, when people say mazel tov, they might also do something else to celebrate, like dance or eat a special treat called a challah, which is a braided bread. It is a happy and joyful time for everyone involved!