The Inside AdSense Blog tells us
We’re constantly thinking about how we can make our ad formats better for you, your users and the advertisers appearing on your pages. Today we’re happy to share the introduction of favicons to our text ads. A favicon is a small graphic that an advertiser can use to brand their website, such as their company logo.
We regularly experiment with small changes to the look of our text ads, measuring the potential impact on user experience and publisher earnings. Our experiments show that adding these favicons to the ads makes advertisers more recognizable to your users, which benefits both you and the advertisers whose ads appear on your sites.