These are patterns of dots that, by using an “app” can be scanned by smartphones and tablets and take the user directly to a web page. This could be text, a video, an audio or indeed anything that can be put on a webpage. By printing these in the book it is possible to show video and play audio fliles to the reader which enriches the experience.
There are many suitable apps, some free some paid. Install just as you would any other app on your device. Click on the app icon then centre the QR code in the box. This should take you to the web page where you may need to click a play icon. All should work OK, best to chose by rating and free/paid.
Apple search App store for “QR”
Windows Store for “QR”