DNS, the Domain Name System, is the directory that turns a domain name into the address of the server that should answer for it. A record in that directory says, in effect, "requests for this name go here." Different record types answer different questions: an A record for a numeric address, a CNAME for an alias, a TXT record for arbitrary text used to prove ownership.
Pointing your own domain at a Birta site means adding one such record where your domain's DNS is managed, not inside Birta itself.
For exactly which record to add, apex versus a subdomain, see connect a domain you already own.