Slang used by hackers and by gamers on multiplayer game servers and in chatrooms. 133t is the slang word for “leet” which is short for elite, meaning the best. 133t speak usually consists of numbers replacing vowels in words (example: A=0, E=3), but occasionally the numbers will replace a consonant. It cannot really be spoken or handwritten as such it is considered to be an Internet language dependent on a keyboard.
Also see All About Multiplayer PC Gaming in Webopedia’s “Did You Know…?” section.