In a checkerboard example where does the (droppable) class come from

In a checkerboard example where does the (droppable) class come from

Checkers
I have been exploring an example in which a checkboard was built. At the beginning of the script I noted the statement:

  1.  $('.checkerboard tr:nth-of-type(even) td:nth-of-type(odd), .checkerboard tr:nth-of-type(odd) td:nth-of-type(even)').addClass('droppable');


My question is: in the addClass method, where does the droppable class come from... there is no reference to it in the program that I am including below...

Also how the piece $('.checkerboard tr:nth-of-type(even) td:nth-of-type(odd) is working

You can execute the complete program I am including here:

<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Checkers</title>
<link href="https://code.jquery.com/ui/1.10.2/themes/smoothness/jquery-ui.css" rel="stylesheet">
<link href="http://file54.com/learning_jquery/site.css" rel="stylesheet">
<link href="http://file54.com/learning_jquery/interactions.css" rel="stylesheet">
<script src="https://code.jquery.com/jquery-1.10.2.js"></script>
<script src="https://code.jquery.com/ui/1.10.2/jquery-ui.js"></script>
<script>
$(document).ready(function() {
  $('.checkerboard tr:nth-of-type(even) td:nth-of-type(odd), .checkerboard tr:nth-of-type(odd) td:nth-of-type(even)').addClass('droppable');

  $('.draggable').draggable({
    containment : '.checkerboard',
    revert : 'invalid',
    revertDuration : 250,
    grid : [90, 90]
  });
  $('.droppable').droppable({
     hoverClass: 'highlight',
       accept : '.draggable',
     tolerance : 'fit'
  });
}); // end ready
</script>
</head>
<body>
<div class="wrapper">
    <header>
        JAVASCRIPT <span class="amp">&amp;</span> jQUERY: THE&nbsp;MISSING&nbsp;MANUAL
    </header>
    <div class="content">
        <div class="main">
            <h1>Drag and Drop</h1>
            <table class="checkerboard">
        <tr>
          <td></td><td><span class="piece draggable"></span></td><td></td><td></td><td></td><td></td><td></td><td></td>
        </tr>
        <tr>
          <td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td>
        </tr>
        <tr>
          <td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td>
        </tr>
        <tr>
          <td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td>
        </tr>
        <tr>
          <td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td>
        </tr>
        <tr>
          <td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td>
        </tr>
        <tr>
          <td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td>
        </tr>
        <tr>
          <td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td>
        </tr>
      </table>
        </div>
    </div>
    <footer>
        <p>JavaScript &amp; jQuery: The Missing Manual, 3rd Edition, by <a href="http://sawmac.com/">David McFarland</a>. Published by <a href="http://oreilly.com/">O'Reilly Media, Inc</a>.</p>
    </footer>
</div>
</body>
</html>

A lot of thanks for explanations.