The Listserver is the automated controller for your Mailing Lists. You talk to it by sending email to Listserver@yourdomain.com. (You can change the name of Listserver on the Misc Settings page).
It has a similar role to "listserv" and "major-domo" that you may have seen with other mailing lists. However, it doesn't quite conform to their standards (they are quite complex), so we didn't call it one of those names in case it caused confusion.
Listserver interprets any messages sent to it and processes any commands that it recognises. You place these commands into the body or Subject of the message sent to Listserver.