FQDN (Fully Qualified Domain Name) is a complete domain name for a specific computer or a host on the internet. It uniquely describes the internet location of the server in the tree hierarchy of Domain Name System (DNS). For example, www.google.com is an FQDN for the site Google, where www is the host, google is the second-level domain and .com is the top-level domain. As seen in the example, FQDN specifies all the domain names which are separated from each other by dots, starting with the host and going all the way up to the top-level domain.

When you want to connect to a host, you generally provide its FQDN to the browser, like www.google.com. The DNS server then resolves the host to its respective IP address by looking into the DNS table. The host is then contacted and you get connected. The FQDN of a host is unique and the order of the domains is very important. As we move from left to right, each section represents a more general level in DNS pecking order.
The final dot following the FQDN represents the DNS root domain, which is unnamed. DNS resolvers qualify any domain name having a dot as a FQDN and append the necessary trailing dot to it, if it is missing. If the domain name does not have any dots, then the resolvers append it with the default domain name and the final dot. Sometimes, due to incorrect DNS suffix search order in your computer TCP/IP properties or due to a corrupted DNS table, the application you are using may not connect to the server if you provide only the hostname. In this case, providing the FQDN will allow the DNS to locate the server. FQDN also helps when you are trying to connect to a remote host which is not local to your internet service provider (ISP).