to call a menu in may be header.php ======hear all value is default value ========but i can not understand the items_wrap

wp_nav_menu

<?php

$defaults = array(
	'theme_location'  => '',
	'menu'            => '',
	'container'       => 'div',
	'container_class' => '',
	'container_id'    => '',
	'menu_class'      => 'menu',  //this class is for ul="" 
	'menu_id'         => '',
	'echo'            => true,
	'fallback_cb'     => 'wp_page_menu',  //this is from a function there a falll back system define
	'before'          => '',
	'after'           => '',
	'link_before'     => '',
	'link_after'      => '',
	'items_wrap'      => '<ul id="%1$s" class="%2$s">%3$s</ul>',       //i don't understand that
	'depth'           => 0,
	'walker'          => ''
);

?>