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
URL masking is nothing but covering up a URL with a different one. It presents one URL when the actual URL is something different. Causing the browser's address bar to show a URL different than the actual URL of the web page being viewed.
Domain Masking or URL Masking is the art of hiding the actual domain name of a website from the URL field of a user's web browser in favor of another name. Source : Wikipedia