An FQDN, or a Fully Qualified Domain Name, is written with the hostname and the domain name, including the top-level domain, in that order - [host name].[domain].[tld].
In this scenario, "qualified" means "specified" since the full location of the domain is specified in the name. The FQDN specifies the exact location of a host within DNS. If the name isn't this specified, it's called a partially qualified domain name, or PQDN. There's more information on PQDNs at the bottom of this page.
An FQDN might also be called an absolute domain name since it provides the absolute path of the host.
FQDN Examples
A fully qualified domain name is always written in this format: [host name].[domain].[tld]. For example, a mail server on the example.com domain may use the FQDN mail.example.com.
Here are some other examples of fully qualified domain names:
www.microsoft.com
en.wikipedia.org
p301srv03.timandtombreadco.us
Domain names that are not "fully qualified" will always have some sort of ambiguity about them.
For example, p301srv03 can't be an FQDN because there are any number of domains that might also have a server by that name. p301srv03.wikipedia.com and p301srv03.microsoft.com are just two examples - knowing only the hostname doesn't do much for you.
Even microsoft.com isn't fully qualified because we don't know for sure what the hostname is, even if most browsers do automatically assume it's www.