Coolness levels can only be raised in increments of 20%, thus things can be 20, 40, 60, etc percent cooler.
Can be used to refer to temperature as well.
Guy 1: Yo man check out Black Ops' gameplay, its amazing!
Guy 2: Meh, its ok, needs to be about 20% cooler.
CNN reporter: The reactors are still overheating and an effort is being made to cool them.
Couch potato: Hmm they need to be about 20% cooler.
It is a term used to call attention to something that is not cool enough, and that it needs to be "20% cooler".
The usage has become popular among the bronies/bronys after airing of Season 1 MLP: FiM's episode, Suited for Success.
-Dude, what do you mean? Is it cool or not?
-Well, it's cool, but not cool enough. It needs to be... 20% cooler.