To snark is to complain, often in a witty or sarcastic manner. In the PPC, we enjoy snarking at badfic, and Mary Sues make us especially snarky (that is, inclined to snark). Therefore, snark is an expected element of PPC missions.
The origin of the word appears to be an imitation of the Low German snarken, North Frisian snarke, and/or Swedish snarka, meaning "to snort." It may or may not have anything to do with Lewis Carroll's "The Hunting of the Snark." Probably not.