Confusions about Wordpress get_the_title function

[bigdaveyj]

The function:\[code\] /** * 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 ) { $post = &get_post($id);$title = isset($post->post_title) ? $post->post_title : '';$id = isset($post->ID) ? $post->ID : (int) $id;if ( !is_admin() ) { if ( !empty($post->post_password) ) { $protected_title_format = apply_filters('protected_title_format', __('Protected: %s')); $title = sprintf($protected_title_format, $title); } else if ( isset($post->post_status) && 'private' == $post->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 );}\[/code\]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?\[code\]$protected_title_format = apply_filters('protected_title_format', __('Protected: %s'));\[/code\]