FQDN Setup help

Ok so sorry for the super Noob question, I can’t seem to find a how-to. Can someone point me in the right direction to set up an FQDN. I own a domain name,currently at GoDaddy not being used. I want to point unifi.domain.com to my unifi stack at my house, how do I do that. Everyone always says make sure to use an FQDN but no one ever explains what and how to set up an FQDN.

Again sorry for such a noob question

Godaddy has an article on how to do it here https://www.godaddy.com/help/create-a-subdomain-4080