Isset in php - how it works

I´m learning PHP with book. And I found this:

 <?php 
         if(!isset($_GET['do']) || $_GET['do'] != 1) {
         switch ($_GET['type']) {
           case 'movie':
           echo 'are you sure?</br>';
           }
           echo '<a href="' . $_SERVER['REQUEST_URI'] . '&do=1">yes</a>';
           echo 'or <a href="admin.php">no</a>';
         } else {
    ?>

What is if(!isset($_GET['do']) || $_GET['do'] != 1) doing and why is it on the first line in script? Thanks!