title: "icon-bar" in twitter bootstrap navigation bar
body: <p>I cannot understand what the following code means in terms of <code>icon-bar</code>:</p>
  
  
  
  <pre class="lang-html prettyprint-override"><code>&lt;button type="button" class="navbar-toggle" data-toggle="collapse" data-target=".navbar-collapse"&gt;
    &lt;span class="icon-bar"&gt;&lt;/span&gt;
    &lt;span class="icon-bar"&gt;&lt;/span&gt;
    &lt;span class="icon-bar"&gt;&lt;/span&gt;
  &lt;/button&gt;
  </code></pre>
  
  <p>What is <code>icon-bar</code> for? Why are there three similar instances of it?</p>
  
  <p>This code is in the navigation bar section: </p>
  
  
  
  <pre class="lang-html prettyprint-override"><code>&lt;div class="navbar-header"&gt;
    ...
  &lt;/div&gt;
  </code></pre>