1. The title of an awesome movie.
2. The title of an awesome song.
3. What it means is to walk the line when police are checking if you are sober. To be able to walk the line means you got your shit straight and not some bum that doesnt know what to do. To understand it better listen to the song or watch the movie.
June: John your so drunk you cant even walk the line.
John: Oh..I can walk it. I can walk it just fine*hickup*then falls on his ass*