This is the default page that is set for a domain when the domain is registered with TransIP and is not pointed to a website.
If you are the owner of the domain, you can point the domain name to your own server in the DNS settings of the domain. This message will then disappear.
If you have a web hosting package with the domain, it is important that you use the DNS settings of your web hosting package and upload your website files to the www folder. This message will then disappear.
The /www/ folder is always the default folder on a web hosting package where your website is stored. This folder is called the DocumentRoot or website path. If your website files are stored in a different folder than the www folder, you can also change the DocumentRoot to the correct folder.
Do you see the reserved page instead of the website you expect? In that case, the owner probably recently transferred the domain to TransIP but has not configured it correctly yet. You can try again later or contact the domain owner through another means.