The comment meta data is the information you provide to viewers about each comment. This information usually includes the author of the comment, when it was written (or posted), etc. In some cases, some comment meta data information become orphan and do not belong to any comment. They are then called “orphan commentmeta” and should be cleaned since they are not useful.
To clean these orphan commentmeta, you can either use our plugin Advanced Database Cleaner, or execute the following sql code (which is used by the plugin) directly via your phpMyAdmin:
DELETE FROM commentmeta WHERE comment_id NOT IN (SELECT comment_id FROM comments)