A hostname is any name placed to the left of your domain name which resolves to an IP address or other FQDN
For example in the FQDN www.domainindyra.com "www" is the hostname.
The word hostname is used because on a local network each machine (host) is usually assigned a name to make it easily identifiable. The same principle is applied to domain names, where a name is applied to save the need to remember an IP Address.