ON THIS LINK

The WordPress developers, fortunately, thought of this.  In fact, they store all kinds of stuff that they don’t want the user to see in custom fields – things like the last time the post was edited, who is currently editing it, and a few others.  A quick look at the database, reveals this:Notice a trend?  The mysterious custom field key values are prepended with an underscore.  Give it a try – enter a new custom field from the edit-post page, and enter a name that starts with an underscore – like _thumbnail, or _meta_keywords.  Hit “Add Custom Field”, and it disappears – but if you check the database, its right where it should be. Now get out there and start hiding things from your users!