Hi,
It is in Hebrew and the meaning is: Mazal = luck Tov = good so it's congratulating someone to have "good luck" but as Eliesheva mentioned (gave her 2 thums up) it is used in different ways, e.g: Achieving something after many attempts, new born, weddings, Bar-Mitzva, after signing a deal / contact, new job, new position, etc.
Best regards,