Cliff questions are questions on the cliff of being deleted by the automatic cleanup. See http://meta.stackoverflow.com/questions/78048/enable-automatic-deletion-of-old-unanswered-zero-score-questions-after-a-year/92006#92006 for more information on this cleanup. Often, it will take just one or two downvotes to push it over the cliff into deletion. Frankly, there's a lot of crap just sitting on the edge that isn't helping anyone, but got an upvote somewhere along the way.
Q&A about the site for administrators, end users, developers and designers for Craft CMS