title: What does this line of code in UI sortable mean?
body: <p>I am using the UI jquery sortable plugin and have found some code that I am going to use but there is one part I don't understand. It is "onChange: "function(serialized) { widgets_positions(); }"</p>
  
  <p>onChange doesn't appear in the documentation. Widgets_positions is a function that I understand about which tracks the positions of the objects being moved around. But I need to understand the 'onChange: function(serialized)' part.</p>
  
  <pre><code>                $('#col').Sortable(
                      {
                          accept: 'widget',
                          opacity: 0.5,
                          helperclass: 'helper',
                          onChange: function(serialized) { widgets_positions(); },
                          handle: '.widget_title_bar'
                      }
                  );
  </code></pre>