QR codes are like barcodes, but they use two axes to get information.
If you know how a barcode works, imagine that, but with an x axis and a y axis instead of just an x axis.

To read them, find an app or whatever that uses your camera to decipher and read the data in a QR code. Usually you have to take the QR code, and fit the edges of it with the markings on the screen for it to read properly.