What is a default domain?
A default domain is the preferred domain of the website that is loaded. This means that, for example, it could be demanded that the website www.checkdefaultdomain.com must be loaded as checkdefaultdomain.com, in other words without the "www". It's also possible that a website has moved to a different domain name. By using a default domain, the webmaster can demand that the new name is loaded as the definitive name.
A default domain is also important for web crawlers of search engines. If no clear default domain has been enforced, it's possible that the same pages are indexed on several domain names. This causes duplicated content, which goes against the rules of the search engines and can therefore result in a lower ranking of the website in the search engines' search results, or possibly even the website result being removed from the search engine altogether.