It means, "It's our thing" (i.e., the Mafia). Puzo put those words in Vito's mouth (in the novel) at the Great Gangster Convention. He picked it up from Joe Valachi, who disclosed it publicly several years before Puzo wrote the novel.
Ntra la porta tua lu sangu � sparsu, E nun me mporta si ce muoru accisu... E s'iddu muoru e vaju mparadisu Si nun ce truovo a ttia, mancu ce trasu.
