The QR code pet tag app works by assigning a unique QR code to each pet tag. When a pet goes missing, anyone who finds the pet can scan the QR code using their smartphone or QR code reader app. The QR code then instantly connects them to the app, where they can access vital information provided by the pet owner, such as contact details, medical history, and photos. This allows for quick and efficient reunification of the lost pet with its owner.