Apple Pay offers an easy, secure, and private way to pay on iPhone, iPad, Apple Watch, and Mac. Users can pay with a credit card stored on their Apple Pay Wallet using an iPhone, iPad or Mac with Touch ID. On older Macs (without Touch ID), payment can be made with a nearby iPhone or Apple Watch (which must be paired with Mac via Handoff). The user must be logged in to their iCloud account on all devices and the devices should be running on iOS 10 or macOS Sierra.
How it works:
From a Mac: Select ApplePay at checkout. After submitting your order you will be asked to put your finger/thumb on your iPhone or iPad to authorize the purchase (your iPhone/iPad must be near your Mac). After authenticating yourself with your fingerprint, your order will be submitted (takes 10-12 seconds).
From an iPhone/iPad: Select ApplePay at checkout. You will be asked to immediately authenticate yourself and complete your purchase.
From a new MacBook Pro with Touchbar: Select ApplePay at checkout. You will be asked to press your finger on the touchbar to authenticate yourself and complete your purchase.
For full details on Apple Pay, visit Apple's support page.