This is a copy from the ZXing repo wiki pages at commit hash 3110236. ZXing is an open-source, multi-format 1D/2D barcode image processing library implemented in Java, with ports to other languages.
Much has been said about 2D barcodes, and the discussion has focused on the format of the 2D barcode itself – QR Code, Data Matrix, and so on. But equally important is the format of what the barcode itself encodes.
2D barcodes encode text, generally, but that text can represent many things. Commonly, 2D barcodes encode text that represents a URL, like https://google.com/m. This is a special string of text since it is recognizable as a URL by readers, and therefore can be acted upon: the reader can open the URL in a browser.