Use value of post meta for something when the post meta changes

post meta

/**
 * Use value of post meta for something when the post 
 * meta changes
 * @param  integer $meta_id    ID of the meta data field
 * @param  integer $post_id    Post ID
 * @param  string $meta_key    Name of meta field
 * @param  string $meta_value  Value of meta field
 */
function saveYouTubeInfo($meta_id, $post_id, $meta_key='', 
                                             $meta_value=''){

    // Stop if not the correct meta key
    if ( $meta_key != 'my_meta_field_name') {
        return false;
    }

    // Function code goes here.
}

add_action('updated_post_meta', 'saveYouTubeInfo', 10, 4);