InterWiki links are a quick way to refer to information in other wikis, without having to remember long or cryptic URLs. To create an InterWiki link, just use the InterWiki name of the foreign wiki, then a colon (:), followed by the name of the page on that wiki.
WikiWikiWeb:InterWiki says: Historically, the term referred to having one wiki, distributed across several servers; See: Distributed Wiki, One Big Wiki (Switch Wiki), Wiki Web Transfer Protocol, World Wide Wiki Web, Multi Server Wiki Over time, InterWiki has come to mean connecting existing wiki, of various WikiEngine types, together. cf Federated Wiki
You can add new entries by editing InterWikiMap.
Of course, InterWiki links are not rendered differently when the remote page does not exist, and MoinMoin cannot check whether you followed the rules for Wiki Name``s of the other wiki. So double-check what you type.