Put simply, nameservers are servers on the internet that answer any queries to your domain and tell the connecting client which machine they need to access. It is therefore very important that these are set correctly, or your domain will not function.
Standalone Domain Registration
If you are using the domain with another hosting company, they will be able to give you the nameservers required. If however, you are registering just to "keep a name" or wish to use either of the forwarding services or the Dynamic DNS then leave this entry blank and they will be filled in for you automatically on registration.
Registration for use with an existing iNETFX Hosting Account
If you are registering the name to use with an existing iNETFX account, then you can find the nameservers you will need to use by logging into your hosting control panel and selecting the domain name. The nameservers will then be listed on the resulting screen.
Registration at the same time as purchasing hosting
As with the standalone registration, leave the nameserver fields blank and as part of the signup process, they will be set for you automatically.