Confusions about Wordpress get_the_title function
<p>The function:</p>

<pre><code> /**
 * Retrieve post title.
 *
 * If the post is protected and the visitor is not an admin, then "Protected"
 * will be displayed before the post title. If the post is private, then
 * "Private" will be located before the post title.
 *
 * @since 0.71
 *
 * @param int $id Optional. Post ID.
 * @return string
 */

function get_the_title( $id = 0 ) {
    $post = &amp;get_post($id);

$title = isset($post-&gt;post_title) ? $post-&gt;post_title : '';
$id = isset($post-&gt;ID) ? $post-&gt;ID : (int) $id;

if ( !is_admin() ) {
    if ( !empty($post-&gt;post_password) ) {
        $protected_title_format = apply_filters('protected_title_format', __('Protected: %s'));
        $title = sprintf($protected_title_format, $title);
    } else if ( isset($post-&gt;post_status) &amp;&amp; 'private' == $post-&gt;post_status ) {
        $private_title_format = apply_filters('private_title_format', __('Private: %s'));
        $title = sprintf($private_title_format, $title);
    }
}
return apply_filters( 'the_title', $title, $id );
}
</code></pre>

<p>I don't understand what the parameter <code>__('Protected: %s')</code> means in a particular line of code below. What kind of parameter is it?</p>

<pre><code>$protected_title_format = apply_filters('protected_title_format', __('Protected: %s'));
</code></pre>
