This is MediaWiki Extension:TwitterCards
Twitter describes this feature like this: Twitter cards make it possible for you to attach media experiences to Tweets that link to your content. Simply add a few lines of HTML to your webpages, and users who Tweet links to your content will have a "card" added to the Tweet that’s visible to all of their followers. It would be really nice for articles, images, videos - everything linked from Wikimedia on Twitter could have a nice little preview on Twitter. The documentation is at TwitterCards - basically it involves adding some meta tags around existing content server-side.
MediaWiki URL : http://www.mediawiki.org/wiki/Extension:TwitterCards