How to update google adsense ad in x seconds

Google even does this, however I do not understand the part of googletag.DefineSlots, I do not know where it gets that numbering. Google Example:

 <script>

    // Load GPT asynchronously
    var googletag = googletag || {};
    googletag.cmd = googletag.cmd || [];
    (function() {
      var gads = document.createElement("script");
      gads.async = true;
      gads.type = "text/javascript";
      var useSSL = "https:" == document.location.protocol;
      gads.src = (useSSL ? "https:" : "http:") +
      "//www.googletagservices.com/tag/js/gpt.js";
      var node = document.getElementsByTagName("script")[0];
      node.parentNode.insertBefore(gads, node);
    })();
 </script>

 <script>

   googletag.cmd.push(function() {

    // Define the ad slot
    var slot1 = googletag.defineSlot("/6355419/Travel", [728, 90], "leaderboard").
     setTargeting("test", "refresh").
     addService(googletag.pubads());

   // Start ad fetching
   googletag.enableServices();
   googletag.display("leaderboard");

   // Set timer to refresh slot every 30 seconds
   setInterval(function(){googletag.pubads().refresh([slot1]);}, 30000);
   });
 </script>