Bob, a software architect, writes, “The diagram is part of the key used on the Pioneer 10 and 11 plaques and the Voyager 1 and 2 records. It represents the spin-flip transition of neutral atomic hydrogen, and so provides a universal physical constant, a measure of length and of time, by virtue of the photon it emits. These are the base units on those plaques and records, where we attempt to communicate very precise information without any shared language or other common assumptions. This diagram is part of the ultimate ‘message in a bottle.'”
Carl: More on the spin-flip diagram at Wikipedia.