JavaScript Random Number Function Script Build

I am new to JS, and I am looking for a line by line walk through for an example I am working in my JS textbook. Any help will be appreciated.

<script>
      var total = 0;
      for (i=1; i<=5000; i++) {
        var num = Math.random();
        total += num;
        if (i % 1000 == 0) {
           document.write("Generated " + i + " numbers...<br>");
         }
      }
      var average = total / 5000;
      average = Math.round(average * 1000) / 1000;
      document.write("<h2>Average of 5000 numbers is: " +
                     average + "</h2>");
    </script>