Have you noticed that QR codes are everywhere nowadays? We can find QR codes in cinema, restaurants, metro, poster, magazines, billboards. The QR codes are readable by machines. Normally people need to use a QR code reader app on their mobile phones to scan them. You can find many QR code scanners or readers on Google Play Store and Apple App Store. They are typically free. Some Android devices, like Xiaomi & Redmi phones, have the built-in support to QR code scanning in their stock Camera apps for years. And finally Apple follows the suit. The iOS 11 has included the QR code scanning functionality in the native Camera app for iOS. You no longer need to download or use a third-party QR code reader in order to scan QR codes on iPhone and iPad.
How to scan a QR code with your iPhone or iPad?
Run the Camera app on your iOS device. Point your camera toward the QR code. Your iPhone or iPad will interpret the content of the QR code, then tells you what the QR code about and what you can do with it.

Generally speaking, QR codes contain website links, so you can scan QR code and quickly visit the site without the need to manually type in the long URL addresses in your web browser. Some QR codes could include name cards or contact information, calendar events, plain text and more. If the QR code you are scanning contains a website link, you’ll get a prompt saying this is a website QR code, you can then tap on this message to open it in Safari browser on iPhone or iPad. If it is a contact QR code contain phone numbers, email addresses, you can just tap to add them to the Contacts app.
Turn on or off QR code scanner on iPhone iPad
QR code scanning option is enabled by default. If you do not like it or you have your favorite QR reader app, you can manually turn it off from iPhone Settings >> Camera >> Scan QR Codes, see below screenshot.

QR codes reading is really welcome feature to the stock Camera app. With this new feature built-in, iPhone and iPad users can scan QR codes with minimal effects, without the need for a separate app. Thus we do not recommend to disable this functionality.
Scan QR codes using third-party scanners on iPhone iPad
For earlier iOS versions before iOS 11, you can download and use a third party QR code reader app instead. For example, you can use WeChat to scan QR code on iPhone. Do a search on the App Store, you will find more free scanner apps.
Very clear, thorough and helpful. Thank-you