This command sets the host name for your services. It will show up when people /whois the nicks for any of the services and also in the messages sent with kills.
/as services set host webmaster.com
Your host should probably be set to something related to your network. This is merely an aesthetic detail, it doesn't affect the actual operation of the chat server, but allows for a more professional or personalized appearance for your network. For other commands that customize the appearance of services see SET USER and SET HOST.