Google Chrome is just one of many Google products that will reportedly be getting support for the Payment Request API in the near future, including native support for implementing third-party Android apps with the API as valid payment methods. The API itself allows for almost any app to tap into the complex framework utilized by mobile payment apps, and allows client-side apps, such as browsers, to verify and accept payments from these third-party apps. Google has the Payment Request API listed as something that they'll be talking about at Google I/O this year, and word is that they will be fully integrating support into Chrome, including the Android version, allowing just about any app that wants to function in much the same way as Android Pay online to do so.
The inner workings of the API are extremely complicated. To put it as simply as possible, the payee's framework first asks the third-party payment app for credentials, then runs them through the app's declared server. If those check out, the payment app spits out a unique key file. If that's a match, then the whole mess connects to a customer's account, a bank, or whatever else may be the backend for the payment app. If everything checks out, the user will be able to choose the third-party payment app to make their online payment, and from there, the payment app double checks the signatures of the web browser or payment accepting app to make sure it's paying who it thinks it's paying, then finally pushes through the payment.
The implications here are fairly vast just in the realm of Chrome, but Google and the World Wide Web Consortium, the creators of the API, want to see it spread further. Essentially, if a customer wants to use their Walmart Pay account to pay for a transaction on eBay, that's something that the Payment Request API can technically do. Likewise, if an otherwise unrelated app wants to implement their own in-app payment method and possibly allow the user to use that payment method in other apps, that's also within the purview of the Payment Request API. For now, seeing the API hit Chrome will be a big first step in getting web developers and merchants on board.