I wonder if you know,
How they live in Tokyo,
If you seen it then you mean it
Then you know you have to go.
Fast and furious! (Kitaa!) (Drift, Drift, Drift)
Fast and furious! (Kitaa!) (Drift, Drift, Drift)
Superficially, it means just what it says, that if you have seen something, then you must mean something. I suppose we can infer it means you are for real, that you actually live a certain kind of life.
In the context of the song -- which I assume is from the movie The Fast and The Furious: Tokyo Drift -- it could mean something like:
If you have made the effort to see the kind of racing they do in Tokyo, that means you are a real racer who truly understands the sport.
But the truth is that it really doesn't mean anything special. "If you seen it then you mean it" is just a kind of cool-sounding filler phrase used in songs which are more about rhythm and mood than about the lyrics. The song itself is designed to appeal to people who like movies about racing and fast cars in "exotic" locations like Tokyo, and no only really listens closely to the lyrics.