Just a plain UPC-A barcode. How it works: UPC codes use 1-to-4 "pixel" wide bars to encode digits; each digit is a fixed 7 "pixels" wide overall (technically, they are called "modules" and not pixels). Wikipedia has the full details, but the encoding is nicely presentable as a simple linear layout.
BarCodeSee the Pen by unframework (@unframework) on CodePen.