The acronym FQDN stands for "Fully Qualified Domain Name" and means the entire hostname, including the domain part. Here are some examples of FQDNs:

www.ardc.net
mail.callsign.ampr.org
this.is.a.very.long.fqdn.com
In these examples, the unqualified hostname would be "www", "mail" and "this" respectively.

In the context of this portal we use the term mostly when interacting with DNS records.

If you are a licensed radio amateur you will most likely want to use our domain name ampr.org and create a subdomain under it using your callsign.

For example, if your callsign is A1BCD then your subdomain would be a1bcd.ampr.org