Original: It runs the browser inside the window space of another application e.g. in Microsoft Outlook, where the page was then made to look like the content one would expect in an email, leaving any passers-by with the impression the person was reading e-mail.
Simplified: The browser runs inside the window space of another program e.g. in Microsoft Outlook, where the page is made to look like the things one would expect in an email, making people think the person was reading e-mail.