AFD = Automatic File Distribution.
 
You have subscribed to the files(s) you specified in the AFD command.
Any time that file (or files, in your case) is updated, LISTSERV will
send you a copy.  In your case, you've subscribed to release notes for
LISTSERV, so when Eric stores V18B RELNOTES (for instance) in the LINFO
file archives, you will automatically get a copy...and V18C RELNOTES,
V19A RELNOTES, etc., as they are stored.
 
The listserver has advised you to set a password so no one can tamper
with your AFD and/or FUI (File Update Information) subscriptions by
sending forged mail or via some other hack attack.  You don't have
to do this if you don't want to.  It's probably a good idea to do so,
but just be sure you remember the password. :)
 
FUI is essentially the same concept as AFD, except that FUI causes
LISTSERV to send a notice that the file has been updated rather than
sending you the file itself.
 
Nathan