The phrase "tu si 'na cosa grande" is in Neapolitan dialect, an Italian regional language. It translates to "you are a great thing" or "you are a big/lovely thing" in English. In the context of the famous song by Domenico Modugno and performed by several artists including Ornella Vanoni and Mina, it expresses deep affection, meaning "you are a great/lovely thing to me" or "you mean a lot to me".
This phrase is often used poetically in songs to convey strong love or admiration for someone.
