A QR code is a two-dimensional matrix barcode read in both axes, with three large finder squares that let a scanner locate and orient it from any angle. That omnidirectional read is why it works on a phone held casually over a scan window, and why self-checkout cameras handle it so well.
It carries far more than a linear barcode: thousands of characters if needed, including letters, URLs and structured payloads. For loyalty that headroom means a pass can encode a member number, a signed token that changes per visit, or a URL that resolves to the member, without running out of space or width.
Built-in Reed-Solomon error correction lets a QR code still read when part of the symbol is obscured, dirty or reflecting light off a phone screen. The correction level is a trade against capacity, and higher levels are worth choosing for codes displayed on glass.
Both Apple Wallet and Google Wallet render QR codes on passes, making it the single format most likely to work across a mixed estate. Where a merchant's scanners are older laser units that read only linear codes, Code 128 is the fallback; see QR and barcode identification for how to decide per site.