It is just the hostname of the machine you are running calibre on. If you don't have one or know how to find it ("hostname" command works on most windows boxes and linux distros) you should be able to use that machine's IP address. For example if your machine running calibre is called jakewalk you would run this from the command line:
calibre-server.exe --hostname jakewalk
Then on your iPod Touch or Iphone you go into Stanza's advanced settings page and set the feed url to be:
http://jakewalk:8080/stanza
Hope that helps.
Cheers,
Crazy