title: What's the meaning of the comma in a jQuery selector
body: <pre><code>&lt;html&gt;
  &lt;head&gt;
  &lt;script type="text/javascript" src="jquery.js"&gt;&lt;/script&gt;
  &lt;/head&gt;
  &lt;body&gt;
  &lt;ul&gt;
    &lt;li&gt;&lt;strong&gt;list&lt;/strong&gt; item 1 -
      one strong tag&lt;/li&gt;
    &lt;li&gt;&lt;strong&gt;list&lt;/strong&gt; item &lt;strong&gt;2&lt;/strong&gt; -
      two &lt;span&gt;strong tags&lt;/span&gt;&lt;/li&gt;
    &lt;li&gt;list item 3&lt;/li&gt;
    &lt;li&gt;list item 4&lt;/li&gt;
    &lt;li&gt;list item 5&lt;/li&gt;
    &lt;li&gt;list item 6&lt;/li&gt;
  &lt;/ul&gt;
  
  &lt;script type="text/javascript"&gt;
  $('li').filter(function(index) {
    return $('strong', this).length == 1;
  }).css('background-color','red');
  &lt;/script&gt;
  
  &lt;/body&gt;
  &lt;/html&gt;
  </code></pre>
  
  <p>Given the HTML above, what's the meaning of the comma in the selector below?</p>
  
  <pre><code>return $('strong', this).length == 2;
  </code></pre>
  
  <p>What will happen if I remove the word 'strong'?</p>