Your server's hostname is the name of your server. Since all of your clients will be able to see it, you will want to be careful to choose one you like.
There are a few things to consider when choosing your server's hostname.
The first thing to keep in mind is that the hostname will need to resolve. In order for this to happen, the first thing that must be true is that the domain it is based on must exist and be under your control. If you do not yet have a domain you would like to use for your server's hostname, you can still order your server with a temporary name and register a domain later, but it is strongly recommended to have a suitable domain as soon as possible.
Once you have a domain you would like to use for the server's hostname, you will need to choose a subdomain within that domain that is not used for anything else. Often, people will choose a subdomain like
server. For example, if the domain being used is
domain.tld then the hostname would be something like
Part of this is to make sure the hostname will resolve to this server, rather than to another server you might have with the same domain. But also, it is much more difficult1) to configure a subdomain for something else if it is identical to the server's hostname.