Short Answer… A QR Code, sometimes referred to as a 2D Barcode, is a way to embed data in a graphic that can be read by a scanner. They’re a lot like regular barcodes, but they can hold more information. Basic text, links to websites, phone numbers, contact information, wifi indentification, basically anything that can be boiled down into text can be put into a qr code.
Long Answer… Wikipedia.
Basically all you have to do is open the app, point your camera at the code, and voilá! your phone will take the appropriate steps depending on what the code contained.
Might we suggest the following?