What does 'register nameserver IP's at registrar' mean?

There is a step that must take place before you point your domain at your nameservers; the nameserver names themselves must be registered at the parent servers. What this means is that you must look for a link similar to "Register Nameserver" within your registrar's web interface. This step is necessary because the parent nameservers need to know the IP address of your nameservers before they'll be able to direct DNS traffic at them; since your computer will be handling the DNS traffic for your domain, the only way requests can be directed is if the nameserver IP's for that domain exist at the parent servers. This is why it must be done at your registrar.