Apple Pay on Your Mac
With the new Apple Pay on Mac capability websites you visit will now allow you to make payments using Apple Pay. This is something that websites will need to enable, so you won't see it at every store.
When this is set up, you can click on Pay with Apple Pay and then you can authenticate the purchase on your iPhone or Apple Watch. This adds a layer of security to your purchase since you will use Touch ID on your iPhone or by double clicking the side button on your Apple Watch.