It is used to refer to an attractive female. Usually, it must be a neighbor of yours for it to even make sense since you are referencing the movie.
Guy 1: "I have a new neighbor that just moved in."
Guy 2: "And you're telling me this because?"
Guy 1: "Dude, she's a 406!"
Guy 2: "Have you met her? Can I meet her? Let's kick it at your place right now!"