Bing rewards you for using their search engine and discovering what is new with Bing.
Once you have built up enough points for the gift card you want. You redeem them for the free gift cards.
If you already searching the way why not make it worth your while?
Once you start using discovering Bing they will give you a referral link which you need to keep posted somewhere whether it be in a blog or on your computer to give to other people to use the referral link to earning points. If you don't save the referral link you will lose because the referral link is a limited link in your account with Bing under the discover tab.