URL Masking means that the 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 orignal domain name and not the destination URL
For eg If your website is hosted at http://www.megrihost.com/Virtual_Pr...ps_servers.html. You choose to forward it to http://www.megrisoft.com/services/ ...private_server/ . Then if you enable URL Masking, your visitors will never see http://www.megrihost.com/Virtual_Pr...ps_servers.html as a URL in their browser address bar.

How does URL Masking work?
In case of URL Masking the Domain Forwarding server sends the client 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 client sees the destination page.