It's a QR code generated right on your order's own page that simply encodes a link back to that same order — scanning it (e.g. with a phone camera) reopens that exact order page. It appears automatically on every order and isn't tied to any special module. If the business has also turned on barcode printing for the order page, a barcode showing the order number appears just below it. As a customer you generally don't need to do anything with it; separately, staff also generate their own QR codes on printed bag/delivery/pickup labels for internal handling.