Primary DNS is the main domain name service provider for your domain name, think of it as the master as that is what it's actual technical term is.
Secondary DNS is the copy of the Primary DNS and is used as an alternative to your Primary DNS server.
If you have hosting with ourselves you will notice in the Welcome email it would have provided you with the Primary DNS and Secondary DNS IP address and host name for you to use when updating your Domain Name Deligation, if your domain name was purchased or transfered to Direct Support your deligation should already be set.
