If you enable URL Masking, then 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 example:
If your website is hosted at http://www.tripod.com/12345/YourSite/index.html. You choose to forward http://www.some-domainname.com to this website. If you enable URL Masking, your visitors will never see http://www.tripod.com/12345/YourSite/index.html as a URL in their browser address bar.