Domain Masking or URL Masking is the act of hiding the actual domain name of a website from the URL field of a user's web browser in favor of another name. There are many ways to do this, including the following 2 examples.
Once the URL is masked it displays the URL mask rather than the original URL/domain name. With masking the domain name it does not affect the content of the actual website, it only covers up the original URL/domain name. Domain masking prevents users from being able to see the actual domain website, whether it be due to length or privacy/security issues