It's where you're effectively banned without being told about it. You can still use the service, but any action you take is isolated from the rest of the site. The term I think comes from reddit but the concept is by no means unique to them: your upvotes or downvotes wouldn't affect rankings and nobody would ever see your submissions or comments other than you. The idea is to let spammers or fraudsters waste their energies on accounts that have already been flagged, instead of notifying them that they've been banned so they switch to a clean account.
