It means a free weekend of Xbox Live.
And to those who say Left 4 Dead is nothing new, read the actual previews. It isn't done justice at ALL by short synopses. It most definitely NOT just '4 guys running around shooting zombies'. You're less of a hardened fighting team and more like a ragtag group of '28 Weeks Later' refugees, and it SHOWS! GODDAMMIT I WANT THIS GAME!!!!!!!!