Regarding "The Godfather", the quote is what Michael Corleone tells the father of the girl he is interested in. It means, "If you tell anyone that I am here, you will be killed. If you don't, and introduce me to your daughter, I shall become your son-in-law".
The fuller context is:
In the Sicilian countryside, Michael is being protected by Don Tommasino (Corrado Gaipa), Corleone's partner in the olive-oil import business, and he is closely watched by bodyguards because Santino (Sonny) has informed them from New York: "Your enemies know you're here." He is struck by the beauty of one of the peasant girls Apollonia (Simonetta Stefanelli), described by one of his bodyguards: "I think you got hit by the thunderbolt." At a cafe, Michael tells her father Vitelli (Saro Urzi) that he meant no disrespect or offense to him or to his daughter when describing her beauty:
I apologize if I offended you. I am a stranger in this country. And I meant no disrespect to you, or your daughter. I am an American, hiding in Sicily. My name is Michael Corleone. There are people who'd pay a lot of money for that information. But then your daughter would lose a father instead of gaining a husband. I wanna meet your daughter with your permission and under the supervision of your family with all respect.
After being permitted an introduction to her at Vitelli's home, Michael presents a gift of a necklace to Apollonia and they quickly develop a romantic friendship.