QR code is a square pixelated pattern picture that when scan with a reader app, will redirect you to a website or to provide further info about a product. It is not common in the USA and Europe, but is a dominant method in China to link the offline world to the Internet.
In iOS 11, Apple build a QR scanner directly into the Camera app. To use it, simply point the camera to the QR code. Once a QR code is detected, a popup will appear on top of the camera view window.
Tap on the popup window to navigate to the link embedded in the QR code.