QR is short for Quick Response (The codes can be read quickly by a cell phone). They are basically advanced barcodes that can hold much more data in them (e.g., a website URL, geo coordinates, and text). They are used to take some information from a static place -- a sign, a poster, an ad, or a library catalog -- and put it into your cell phone. You scan it with your mobile device/phone’s camera.
If you have a smartphone or mobile device, go to its app store and search for a free QR code reader (there are several options). Launch the app and then point your phone’s camera at a QR code. Most QR codes have a URL in them, so once you scan it your device will open a browser and take you to the website.