Someone clever came up with something slightly original to say (I'm guessing woot is derived from 'woohoo') and then a bunch of interweb cunts stole it from him.
Starting back from the hacker days when Unix coders use to find the "root" of someones computer and "win" access to it they called it "win root" later evolving in online chat rooms to Woot and spilling into the game community, today is one of the many ways gamers say "Yay, yippy, and yeehaa". A way to express your excitement.