With this option, you will be able to use your own domain name such as "mydomain.com" instead of "mydomain.yclas.com."
After creating your site, you can go to your site details, and from there you can click on a button called "custom domain." Follow the instructions detailed there.
"WWW." it's a subdomain, your domain is "http://yourdomain.com/ " if someone mistakenly decides to type the three extra "WWW," he will be redirected to the real domain "http://yourdomain.com/" so you won't lose any visit. This is much better and easier because the "WWW" does not mean anything.