Fully qualified domain name—the complete address of a host on the internet, from the leftmost label to the root. Example: mail.bunkerdomains.com. (Not just bunkerdomains.com; that's incomplete if you're routing mail or pointing a service at a specific subdomain.)
FQDN matters because DNS resolvers need the whole chain to work correctly. When you register a domain, you own the second-level part (bunkerdomains); subdomains are yours to create and point wherever you want via A, AAAA, CNAME, or MX records. An FQDN includes all of it.
Why this matters to you: if you're running a mail server, VPN endpoint, or hidden service, you need to know the exact FQDN you're publishing in DNS. Misconfiguration—like publishing a naked domain instead of the full FQDN—breaks mail delivery, TLS cert validation, and service discovery. Also relevant if you're setting up reverse DNS (PTR records point to FQDNs, not partial names).
Related to TTL, zone files, and DNS architecture generally. If you're anonymously registering and self-hosting, you'll be managing FQDNs directly in your nameserver config.