URL Masking means that visitors to the domain name will be forwarded to your destination website, however the URL in their browser address bar will remain the same. Also known as Stealth Redirection or URL Hiding. This will ensure that your visitors see the original domain name and not the destination domain name.
Your website is hosted at http://destinationdomain.com and you choose to forward http://yourdomain.com to this website. If you enable URL Masking, your visitors will never see http://destinationdomain.com as a URL in their browser address bar.
URL Masking will also work perfectly with Path Forwarding. In this case, if you type in http://yourdomain/index/index.html in the browser, the URL in the browser would not change but the page displayed will be of http://destinationdomain.com/index/index.html
How does URL Masking work?
In case of URL Masking, the Domain Forwarding Server sends the visitor a frames page where the frame source contains the destination URL you have specified. This ensures that the URL in the address bar of the browser does not change though the visitor sees the destination page.