A Super Critter is someone who makes an outstanding contribution to CC through their critiquing activity. You will see them marked in their member page and in the Story Queue with a star.
To become a Super Critter you must write at least 12,000 words in critiques in a single month. Once you achieve this you will get a Super Critter star next to your name until the end of the next month (this means that if you reach the threshold in the middle of the month you will have the star for one and a half months).
On your dashboardyou can see your Super Critter progress.