jabber-username is the username part of your JID.
jabber-server is the JID of your server, i.e. the hostname part
of your JID. This is usually, but not necessarily, the same as the
hostname of the server.
jabber-password is your password. You have the option to set
it here, in which case it will be stored in cleartext in your
.emacs file. If this is set to nil, you will be prompted for
your password every time you connect.
jabber-resource is the resource you want to log in under. This
only matters if you are connected to the same account from different
clients or different computers, since each connection must have a
unique resource. You might want to set this to your hostname.
jabber-default-priority is the default priority sent with your
presence. Regardless of what you have here, you can change your
priority during a session with jabber-send-presence.
See Presence, for more information on priority.
jabber-nickname is your default nickname for groupchats.