A domain apex is the "root" level of your domain. For example, let's say you just purchased mygreatbrand.com. We'd call that the "domain apex", meaning that mygreatbrand.com is the "root" of the hierarchy of domain names. Any subdomain of that (e.g. www.mygreatbrand.com, mail.mygreatbrand.com, etc.) are not considered domain apexes, but are considered subdomains of mygreatbrand.com.