InterWiki links are a quick way to refer to information in other wikis, without having to remember cryptic URLs. To write an InterWiki link, just use the InterWiki moniker of the foreign wiki, followed by a page name on that wiki. 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 WikiNames of the other wiki. So double-check what you type. See the wiki:MeatBall/InterWiki page on wiki:MeatballWiki for further details.
MoinMoin marks the InterWiki links in a way that works for the ColourBlind and also is LynxFriendly by using a little icon with an ALT attribute. If you hover above the icon in a graphical browser, you'll see to which Wiki it refers. BTW, the reasoning behind the icon used is based on the idea that a WikiWikiWeb is created by a team effort of several people.