technically means extremely or very cool, but most people who say it never just say cool, so its pretty much useless, unless you can use it with moderation.
guy1: lol, i just totally ownage pwnd these n00bs.
guy2: thats so uber cool, you should kill yourself.
Uber comes from german for super. Cool comes from english for not hot. Together they indicate either an extremely low temperature, or something's coolness exceeding normal limits.