Barcodes are everywhere. They provide a uniform interface for machines to identify real world items. Thanks to the decimal code underneath the bars, barcodes are also interpretable by humans, which makes them wildly used virtually everywhere.
Barcodes are, perhaps surprisingly, a form of binary code. They consist of multiple lines stacked vertically. If a line is black its value is 1, where blank stands for 0. Thicker lines are just series of sequential 1’s.