Bing’s “Best match” feature deliver the very best search results. Best match helps you to identify which result might be most useful, particularly when one link clearly provides the official or authoritative source of information. It displays the result in a clearly marked area at the top of the page, and includes deeplinks into the most helpful parts of the site right beneath the site description. Best match can include a “search within” bar that allows you to set up a query of the official site. Best match also shows similar or related sites to your search, to allow you to explore around the actual result to your query.
More info: Bing Search Blog