I was submitting a product to Clickbank and got this response.

You are welcome to use the brand name as an extension, but it cannot be used in the actual domain. An example of an acceptable URL you could use would be, mydomain.com/twitter.

What is an extension? A page of the main domain?

So if I wanted to do as they suggested I could add this page to another domain and submit that URL?

Thanks!

A page of the main domain?
I suppose that is mydomain.com and in this case .COM can be called as extension. In any case I think that you need to place more infor with your question. Correctly asked question contain half of the right answer ;)
Good luck

Hello,

Yes, the domain name extensions are - .com, .net, .info, .org, .mobi etc...

Regards