QR codes are 2D barcodes that are used to contain information on URLs, addresses, text messages etc. They can be read (scanned) using QR code reader software, if it is downloaded to your mobile device or computer. Recently they are used by the libraries to direct patrons with mobile devices to certain URLs with information on virtual tours, catalog pages, to text messages on policies or interlibrary loan information.
You can generate your own QR code by going to QR code generators pages (just type in "QR code generator" using Google). Easy to use sites that generate QR codes for you: qrcode.kaywa.com and delivr.com/qr-code-generator.
The QR Codes site describes very well the usage of QR coding in the libraries.