Apple Pay is a digital wallet from Apple which lets you use a compatible iPhone, Apple Watch, iPad or Mac to make secure contactless purchase in stores, within selected apps and participating websites.
Other helpful FAQs.
No, your card details are not stored on the device or in the Cloud. When a card is added to Apple Pay, a Device Account Number replaces the need for the card number
The Device Account Number is used to make secure purchases in stores and within apps and websites. It will not work if it is compromised in any way and cannot be used outside of the device it is assigned to.
The Device Account Number can be found by opening the Wallet app, tap the card, then tap the (***) symbol in the top right corner and then scrolling down to find Device Account Number.