The observing frequency, 1420 MHz, corresponds to the wavelength of radiation produced by a quantum mechanical "spin-flip transition" in neutral hydrogen atoms. Because hydrogen is the most abundant element in the universe and because 1420 MHz is a relatively quiet frequency free from sources of natural radio interference, many SETI experts think that 1420 MHz is a good place for interstellar broadcasts. Would aliens reason the same way? There's no way to know, but the search has to start somewhere.
But really this isn't about astronomy. It is more my own signal for my family and friends; a way to tether home.