need Code explenation

Hallo I just started coding a wordpress theme for first time and I need some explanation of this code
(I’m following this guide btw)
http://www.wpdesigner.com/2007/02/22/wp-theme-lesson-3-starting-indexphp/

<?php comments_popup_script(); // off by default ?>

Rest of code.

<?php wp_get_archives(“type=monthly&format=link”); ?>
<?php comments_popup_script(); // off by default ?>
<?php wp_head(); ?>