A QR Code is a Quick Response (QR) Code, a type of printed matrix barcode. These were designed for Japan's automotive industry in 1994. You can read more about them here: https://en.wikipedia.org/wiki/QR_code
Codes QR codes have been around for a while, but the lockdown has seen their use catapulted into everyday life. They don't look like much to the naked eye, but they're full of data when read by a computer. The cool thing is that when you use a smartphone's camera to view them, they are programmable. This means they can tell the device to carry out a task like going to a website address, perfect for menus and contact tracing.