Domain masking is simple - the URL shown in the address bar is a custom URL - like www.grabers.com - but the content of the page is from another URL - such as www.xyz.com. There's all sorts of reasons why this could be useful.

This has been around for along time. But what is a lesser known fact is that search engines will use the meta information in the search listings somehow. I hadn't been aware of that and this fact is important.

According to HostingDude.com, "But masking is even more powerful than this! You can do more than just hide the domain name to which your customers arrive. You can also control the Meta Tags which are read by search engines, so that your new website can be found, ranked, and cataloged under the keywords and description you desire.You can insert a Meta Title, Meta Description, and Meta Keywords, and these will be passed to any search engine finding your site. This helps create a separate identity for your site from the site to which you're forwarding, and improves your site ranking. "

This is somewhat open to discussion, but nevertheless still relevant as many search engines still rely on the meta information is some way. Jill Whalen at High Rankings gives an excellent perspective on the current state of affairs in this regard