An apex domain is the root of a domain, example.com with nothing in front
of it, as opposed to a subdomain like www.example.com. The distinction
matters because of a real DNS rule, not a preference: per
RFC 1034, section 3.6.2, a name
that carries a CNAME record can carry no other data, and the apex has to
hold other records too. So the apex can never be a CNAME. It takes an A
record instead.
A subdomain has no such obligation and usually gets a CNAME, which is why hosts often ask for two different record types depending on which one you are pointing.
For the full rule, ALIAS/ANAME workarounds, and which one Birta needs, see connect a domain you already own.