You must enter the domain name. A domain name is a bunch of name glued together with colons. For example
solucorp.qc.ca
can be read as the sub-domain solucorp, part of the sub-domain
qc, itself part of the top-level domain ca.
Unless you have an isolated network, you must request an official
domain name. Say you want to register the domain foo.com, you
will have to request permission to the authority managing the
domain com. If later you want to setup the domain
sub.foo.com, you will have to ask ... yourself, since you
will be the authority of the domain foo.com.
Your Internet provider will help you selecting a proper domain name.
If you are not connected to the Internet, you are free to setup any domain you want.