If you don't know the meaning of the words in the setup sections, you might find the uncompressed version over here.
An alias is another name for your machine. One machine in general may serve several domain names (like a mycitadel.com and mycitadel.org); enter yours here. Don't forget to have a look at your DNS Config
If your citadel is a satellite system (maybe you're behind a dial-in and are not allowed to send out mail directly) you can specify a host which citadel should send your mail out through ("Relay"). You can specify a login here if you like (remember, citadel just supports plain auth) in this syntax username:password@mail.yourprovider.com. You could also want to relay through some other local running MTA (Mail Transport Agent) like Postfix (which will provide more sophisticated client auth mechanisms for example)
A Realtime Blackhole List is usually a DNS that decides whether a domain / ip you ask it for is in suspect of sending spam or not. The FAQ has some examples.
An IP or domain name of a machine you've got the SpamAssassin scanner running, to decide wether you want to receive the mail or reject it at the gates, because it only will try sell you cheap watches.
You've got a Domain where your users should be allowed to send mail from, but that shouldn't be delivered locally? Add them here.