Meta-moderation is a second level of comment moderation. A user is invited to rate a moderator's decision. He is shown a post that was moderated up or down and marks whether the moderator acted fairly. This is used to improve the quality of moderation.
Slashdot and Kuro5hin are two websites with meta-moderation. The GameFAQs message boards used to have it.
References
Meta-moderation system Wikipedia(Text) CC BY-SA