title: What does 'this' keyword mean in a method parameter?
body: <pre><code>namespace System.Web.Mvc.Html
  {
      // Summary:
      //     Represents support for HTML in an application.
      public static class FormExtensions
      {
          public static MvcForm BeginForm(this HtmlHelper htmlHelper, string actionName, string controllerName);
  ...
      }
  }
  </code></pre>
  
  <p>I have noticed that 'this' object in front of the first parameter in BeginForm method doesn't seem to be accepted as a parameter. Looks like in real BeginForm methods functions as: </p>
  
  <pre><code>BeginForm(string actionName, string controllerName);
  </code></pre>
  
  <p>omitting the first parameter. But it actually receives that first parameter somehow in a hidden way.
  Can you please explain me how this structure works. I actually exploring MVC 4 internet Sample. 
  Thank you.</p>