For more of an explanation, here’s a quick except from their blog:

Well, last week, Apple launched iOS 5.1 and with it, an updated Siri and Yelp integration on iPhone. Now when searching for a recommendation on local businesses, Siri will return Yelp-reviewed businesses and when you tap the star rating (BOOM!) you are immediately taken to the Yelp business profile page. Oh, the places you’ll go… now that you’ll have faster access to Yelp review highlights, quick tips, hours of operation, address, phone number, photos — the list goes on.