CNAME is what's known as a Canonical Name Record. It's like creating an alias for one domain and giving it the name of another.
You will need to tell Instapage what domain you want your pages to appear on, and create the CNAME with the hosting provider where the domain is registered. You can check out our article here: https://d.pr/3AUA01 for a step-by-step instructional guide on how to set this up.