If you are a plugin/theme developer and you are planning to use custom fields to store parameters related to your plugin or template, it is interesting to note that WordPress won’t show keys starting with an “_” (underscore) in the custom fields list at the page/post editing page or when using the template the_meta() function. Depending on how you plan to use the meta data, you may want to hide the values from the the admin UI by prefixing their names with an underscore.
The following example:
add_post_meta(68, '_color', 'red', true);
will add a unique custom field with the key name “_color” and the value “red” but this custom field will not display in the page/post editing page.
In addition, when $meta_value is an array, it will not be display on the page/post editing page.