So if you consider that different notes of music are different frequencies of sound, each radio frequency "hopped to" would be a different note on a piano being played on either end of the signal.

From Wikipedia: "Antheil and Lamarr developed the idea of using frequency hopping: in this case using a player piano roll to randomly change the signal sent between the control center and torpedo at short bursts within a range of 88 frequencies on the spectrum (88 black and white keys are on a piano keyboard). The specific code for the sequence of frequencies would be held identically by the controlling ship and in the torpedo. This basically encrypted the signal, as it was impossible for the enemy to scan and jam all 88 frequencies because this would have required too much power. Antheil would control the frequency-hopping sequence using a player-piano mechanism, which he had earlier used to score his Ballet Mécanique."