Private name server or Custom name server is your own unique name server that only use by you. You can create it at your domain registrar and then you will have your own name server, for example: ns1.yourdomain.com, ns2.yourdomain.com
We have include official tutorial of 3 major registrar in this post, Godaddy, namecheap, name.com. If you use other registrar, feel free to contact us for private name server setup guide. We will find it for you.