It means invalid clicks and suspicious impressions, like too many impressions coming from one user. I hope it's safe with our quizzes though. I wonder how the publisher is supposed to get out of a ban like this? How can he prove to Adsense his traffic is valid? Btw, there's a plugin that is supposed to prevent accidental clicks, haven't tried it though. Anyway, I'll be testing my set up with and without this ad.