Short for Jabber Identifier / Jabber ID. Sort of like Jabber's version of ICQ UIN.
Jabber IDs are in format user@host/resource - where user is username, host is hostname of the Jabber server and resource (optional part) usually tells something about the type of the connection (for example, name of the client or place where the user is connecting from.)
For example, I can be reached at wwwwolf@jabber.com, more specifically I'm now wwwwolf@jabber.com/home.