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. its a subdomain, your domain is http://yourdomain.com/ if someone mistakenly decides to type the 3 extra WWW , he will be redirected to the real domain http://yourdomain.com/ so you wont lose any visit. This is much better and easier, the WWW do not mean anything ;)