In 1972, Carl Sagan, Linda Salzman Sagan and Frank Drake proposed a method of using pulsars to identify the location of Earth in the galaxy. An image produced by their method was used on the plaque sent with the Pioneer probes. The same image was used on the Voyager probe’s golden record.
The basic idea is to indicate the distance and direction of several pulsars (in this case 14) as well as their pulse rate. Given that information, one could triangulate the location of Earth in the galaxy, since there would only be one place where the pulsars of the right pulse rate would have the right orientation. In short, pulsars can be used as a cosmic navigational map. Furthermore, since pulsars slow down over time, you could also determine how long ago the plaque was made by comparing the stated pulse rates to the current pulse rates.
Of course, how do you communicate distance and time to a hypothetical alien race that doesn’t know what a meter or second is? Distance is indicated by the length of a line to the pulsar, with a tick mark along that line to indicate the distance above or below the galactic plane. That gives you the direction and distance of each pulsar. For the rate of each pulsar, you express the time in terms of the transition frequency of the 21 cm hydrogen line. This transition occurs at a precise rate of a bit less than a billionth of a second. The rate of each pulsar in these “hydrogen units” is expressed in binary along each line. So in principle, alien scientists could use the image to determine when and where the Pioneer probes originated.
Of course, since 1972, we have better measurements of the distances and pulse rates of these 14 pulsars, so the Pioneer plaque image is not as accurate as it could be. Earlier this year, Redditor hayburg produced an updated version of the pulsar map, seen here. They have even provided the Matlab source code, so you can produce your own image, say corresponding to the location and time of your birth.