What's a fanlisting? What does that 'approved by TFL' means?
A fanlisting is a site which lists fans of a particular subject. In this case, Seven Hells is the fanlisting for Sandor Clegane, and will list fans of this character. TFL is a site that lists fanlistings, and that approved means this is the only fanlisting listed there for the Hound.
So this is not an official fanlisting?
No, it isn't and I'm not claiming so. Seven Hells has no relation whatsoever with George RR Martin and it's not an official fanlisting. This is a fansite made by fans and for fans.
What if I want to be listed?
Take a look at the rules, go grab a code and fill the join form :) If your application follows the rules, you'll be listed in the next update.
Who is Sandor Clegane?
Sandor Clegane, or the Hound, is one of the characters from A Song of Ice and Fire, a fantasy books series by George RR Martin. If you don't know him by now you really should read the books ;) For more info about him you can check this section :D