The C! is a way to reward a noder for submitting a particularly good writeup whether it be factual, humorous or artistic. Basically, a way of saying you appreciated the writeup and wanted to let the author know and give the writeup more attention than it would have had otherwise. C!ing someone's writeup earns it a place in the Cool Archive and the front page. E2 will also reward that user with 20 XP.
You get the power to C! writeups beginning at 4th level. A writeup can accrue any number of C!s but a user can C! any given writeup only once. Further C!s must be given by other users. For details on C!s and the rest of the level system see the voting/experience system document.
Cool Man Eddie will msg you in the Chatterbox every time somebody C!s one of your writeups, telling you who did it. If you don't want Eddie turned on you can uncheck a box in User settings. You can also enable a "cool safety" so you don't accidentally C! somebody's writeup.