WWW stands for World Wide Web, and is a well-known prefix for domain names. To be more precise, WWW is just a subdomain that is commonly used because of the long-standing practice of naming Internet hosts according to the services they provide.
As the use of WWW is not required by any standard or policy, many websites simply don't use it.
However, you can enable or disable the WWW prefix for your URLs in the website's general settings. Toggle the Use WWW Prefix button depending on whether you want WWW automatically added to your URLs or not:
After making the changes, remember to update your website.