A leet speak form of saying huzzah or hurray.
It originated from back in the days of MUD games, after killing an opponent loot was a rare find, so if someone found loot they would say "wow loot!". This evolved into "woot" for short which further evolved to simply be a general exclamation of excitement.
Bob: I just killed a monster! wow loot!
Bob: I just killed a monster! woot!
Bob: Something good just happened! w00t!