What is zone apex?
Zone apex means a simplest domain, which is not a sub domain. For example, if we own a domain "example.com", "www.example.com" is not a zone apex, but "example.com" is a zone apex.
CNAME can not be configured to zone apex
If we use a DNS server outside AWS instead of using Route53, which is a DNS service from AWS, we need to configure the domain name of ELB(ALB) as CNAME record of the domain. However, this configuration may not work correctly because it violates the specification written in RFC1912.
Use an alias record of Route 53
Consequently we can't use outside DNS servers in this case, and have to use the function provided by Route53 which is called "alias record".
Alias records can be configured like CNAME records, and behave like A records. (Route53 will resolve the IP address of the CNAME record and return the IP address as an A record response.)