I came across a very nice explanatory piece for QR codes. If you always wanted to know the basic principles this is a good chance to get a grasp.

QR code (abbreviated from Quick Response Code) is the trademark for a type of matrix barcode (or two-dimensional barcode) first designed in 1994 for the automotive industry in Japan. A barcode is a machine-readable optical label that contains information about the item to which it is attached. In practice, QR codes often contain data for a locator, identifier, or tracker that points to a website or application. A QR code uses four standardized encoding modes (numeric, alphanumeric, byte/binary, and kanji) to store data efficiently; extensions may also be used.^[1]

Wikipedia

I am using QR codes in several of my projects – one example: Miataru uses QR codes to encode the device ID and help with the device handshake. You scan the QR code of your friend with your Miataru client app and immediately will be able to see his location in Miataru. Without the need to enter long rows of numbers.