QR Code
A QR code is a graphical representation of arbitrary data similar to a barcode. A QR code can represent a URL, a Bitcoin address, a Lightning Invoice, and much more. QR codes look like a random assortment of black and white squares, and are usually recognizable by their larger black squares in the corners.
Almost all smartphone cameras are now capable of reading QR codes, so you should be able to point your camera at any QR code and decipher the data.