Private nameservers are where your domain points to our nameservers so that your customers do not see our domain name on your account. If you do not set up private nameserver someone does a nameserver lookup on your domain it will show ns1..com and ns2..com
Once you set up private nameserver when someone does a nameserver lookup on your domain it will show ns1.yourdomain.com and ns2.yourdomain.com which will mask that you are using our services. Your clients can also use ns1.yourdomain.com and ns2.yourdomain.com instead of the default nameservers.
Depending on what registar you used for your domain name you may be able to set up your private nameserver on your own or you may have to contact your registrar to have them set up for you.
If you need any assistance setting up your private nameservers please submit a support ticket and include which domain registar you are using and we will help you get them set up.