title: Default keyword in Swift parameter
body: <p>When reading the initializer for a <code>NSLocalizedString</code> I see that some of the parameters are defaulted to a value <code>default</code>. What does the <code>default</code> keyword represent?</p>
  
  <pre><code>func NSLocalizedString(key: String, tableName: String? = default, bundle: NSBundle = default, value: String = default, #comment: String) -&gt; String
  </code></pre>