The functionality marks all posts and threads that have been reviewed by a moderator, so that other moderators do not need to check the same posts. It also marks unreviewed posts, so that moderators know what is left to check.
Please see this thread about AVForums their post checking system: http://xenforo.com/community/thread...hugely-important-piece-of-functionality.2550/
Over the years I have seen much more advanced versions of the same concept.
Optimally a post checking system works in tandem with moderating statistics, because it allows you to see how much reviewing each moderator does. It would be nice if staff members could see their performance in the side bar, in order to stimulate staff activity.
There is also something to say for the function of allowing certain usergroups to do post checking, so that trusted members can aid the moderators.