Android Pay is Google's answer to Apple Pay and Samsung Pay. Basically you register a credit or debit card to your Android Pay account, and then you can use your mobile device, whether Android watch or phone to pay for items with simply a tap at any store that accepts the Android Pay Service. This is a growing list that includes everything from McDonald's to JetBlue to Office Depot to Macy's and many many more. Google is running a promotion right now to encourage people to sign up. After your 10th tap to pay with Android Pay you can get yourself either a selection from the Google Play Music Store or a free Chromecast. You can see details about the promotion here.
. Overall we'd say if you were thinking of trying out the Android Pay service anyway, this is a nice 'perk' to encourage you to do. The promo runs til the end of February. Happy Chromecasting!