Google's approach to money is many-pronged. There's Google Wallet, Android Pay, the open Payment Request API it's spearheading, and now the brand new "Google Payment API," to name just a few of its approaches.
This latest system will allow you to buy things — both inside apps and on websites — using your Google account. Most people have a credit card stored with Google for some reason or another, like for Google Play Store purchases, or a YouTube Red subscription. The new API will allow third party developers to charge you through your Google account, with Google handling the security and processing.
If this all sounds familiar, it's because Google actually had a product that it killed off back in 2013 called Google Checkout that was very similar. Nowadays developers can use PayPal Express Checkout to accomplish something like this, but the ubiquity of people logged in to Google accounts at all times does give Google a leg up in this space.
Google is also using this same setup in Google Assistant to facilitate transactions, instead of Android Pay. You can say something like "Send $50 to Paul Miller," and Google Assistant will confirm before it fires off the money — using your fingerprint if you’re on Android. Merchants can also get paid through Assistant this way — in Google's demo at I/O they ordered delivery from Panera using the Assistant.
The new API isn't publicly available yet, but Google is already testing it with a few partners. Payments in Assistant will be US-only when it launches in the “upcoming months.”