Referrals is a feature of AdSense that allows you to increase your revenue while increasing your users’ awareness of a wide range of products and services. By adding a referral button to your site, you can direct users to Google products like AdSense and Pack as well as products from our extensive base of AdWords advertisers. When your referral connects a user to one of our referral products, you can generate more earnings while introducing your visitors to new and interesting products.

You can sing up and read more here https://www.google.com/adsense/support/bin/answer.py?answer=43870&topic=8972

Who is it for?

If you have a website with good number of new visitors you may wanna go for it. For sites with little traffic I would suggest stick with other options. That way chances are good that you at least earn something daily.