A QR code is a two-dimensional barcode that stores a small amount of information, usually a web address. Pointing a phone camera at it opens the link without anyone typing anything. Most modern phones can scan one with the built-in camera, so customers and staff need no special app, and codes are cheap to print on labels, signs and paperwork.
In business they bridge the physical and digital worlds. A code on a piece of equipment can open its maintenance record, one on a delivery note can confirm receipt, and one on a site sign can open a check-in form. Take care where a code sends people, and avoid pasting codes over others, because the destination is not visible until it is scanned.