Domain overview & main domain
The domains listed are all domains on which your website can be accessed. Because search engines see multiple domains as duplicate content, we redirect all visitors to the main domain. The main domain will be the address you see in the addressbar of your browser.
This setting indicates if you prefer to see your website with or without www. prefixed. Your website will remain available through either option, we will simply redirect users to the option of your choice.
Traditionally websites always started with www., followed by the rest of the domain name. That custom is starting to disappear, but the usage of www. remaings a question of personal preference.