This is when you mask or hide the actual domain URL from the web browser. An example would be that NO MATTER what page you clicked on (blog, about, faq, domain names etc) it would always say the root domain name (website.com for example) in your web browser. Masking does not impact the websites content in any way, it only covers up the original URL.
That’s Silly, Why Would You Want To Do That?
Some for security purposes, but most people use masking as a way to forward their unused domain names to an already built site. The real estate world is common for this.
Real estate companies will often build domain specific sites for homes or properties. Once those properties are sold, the domain becomes useless. With domain masking you could then forward that domain to another property listing or website.
Rinse and repeat.
The advantage is you can have many domain names all ranking in Google that when clicked on forward to a website (but without changing the URL in the browser).
So to put it another way, you can end up with multiple domain names ranking in Google. Each one without a website of its own (as it forwards to single primary website).
How is This Achieved?
There are many ways to achieve this. The most common way is to frame embed in the main website to point to some other website. You could also perform a URL rewrite or create an alias and have your web server display the same page for two different domains (this one is a bit technical and you’ll need to ask your web host to assist you on this).
Can I mask a sub domain?
Yes, sub domains are typically treated as their own websites as you should only be using them for a specific service not related to the core offering, think maps.google.com as a classic example of a sub domain. To mask, you will want to contact your web host and see what the easiest way to do this is.
Is masking a domain bad for SEO?
Typically yes because of the duplicate content issue. You’re serving up the exact same content for multiple domains. I would avoid masking unless there was a specific reason that warranted using it like a real estate website where you have rankings for property that is no longer available and would rather show a website than do a 301 redirect.