A wallet pass is a digital card that lives in the wallet app already installed on a phone. Apple Wallet on iPhone and Google Wallet on Android are the two dominant wallets, and both ship with the operating system, so a pass requires no app download, no account creation and no plastic.
A pass is more than an image of a card. It carries structured fields such as a member's name, a points balance or a seat number, a barcode or QR code, and optionally NFC credentials so the pass can be read with a tap. Those fields stay live: when a balance changes or a tier is reached, the pass updates in the wallet, and the wallet can surface the change on the lock screen.
Passes come in many shapes. Loyalty cards, membership cards, stamp cards, gift cards, coupons, event tickets, boarding passes, employee passes and pickup passes are all wallet passes with different layouts and behavior. On iPhone the underlying file format is PKPass; on Android, passes are objects created through the Google Wallet API and delivered with a save link.
For a loyalty or membership program, the pass is the always-present alternative to an app the customer rarely opens and a plastic card they left at home. It identifies the member at the till, by NFC tap or QR/barcode scan, and works as a messaging channel for as long as it stays in the wallet.