The post metadata is the information you provide to viewers about each post. This information usually includes the author of the post, when it was written (or posted), and how the author categorized that particular post. In some cases, some post metadata information becomes orphan and do not belong to any post. They are then called orphan post meta and should be cleaned since they are not useful.
You can either use our plugin Advanced Database Cleaner to clean-up these orphan post meta or execute the following SQL code (which is used by the plugin) directly via your phpMyAdmin:
DELETE pm FROM postmeta pm LEFT JOIN posts wp ON wp.ID = pm.post_id WHERE wp.ID IS NULL