Well, you'd use it in a new group you'd create, vs one of the standard/default ones; it's for cleaning up permissions; say you didn't want anyone to use PT's, then you'd create a new group named restricted; and make it a default registration group, then you can open it up to users by removing them from that group.
As for fixing the permission, I'm not sure which perm table it'd be, you may try seeing if "forums_CP_PERMISSIONS" has any -1's in it, if so, edit it and set it to 0
