As one book puts it, if there were a
Pythagorean Music of the Spheres, this would be its opening note.
1420 MHz is the frequency of precession of the single orbital electron of the hydrogen atom. Thus, it is a frequency that is constant wherever there is hydrogen (i.e. everywhere). This frequency is being used by SETI, because it is thought that all fairly advanced civilisations will know of this magical frequency. No matter what units of measurement they use, it will mean something to them.