The phrase is written on a card kept in Slappy the Dummy's front pocket. Once the words are said, Slappy comes to life. If the words are read aloud again, he goes back to sleep. This phrase is used in all of the Night of the Living Dummy books. The phrase also appears in the book Escape from the Carnival of Horrors.
The saying means "You and I are one now".
The phrase is of an unknown foreign language. Google Translate identifies the language as Maltese, but it does not translate to "you and I are one now" in English, meaning that it is mostlikely supposed to be an obsolete ancient language, or perhaps just a magical spell.