Here is a simple explanation taken from Heddy Lamar's biography

During World War II, Antheil and Lamarr discussed the fact that radio-controlled torpedoes, while important in the naval war, could easily be jammed by broadcasting interference at the frequency of the control signal, causing the torpedo to go off course.[SUP][13][/SUP] Lamarr had learned something about torpedoes from Mandl. Antheil and Lamarr developed the idea of using frequency hopping to avoid jamming: using a piano roll to unpredictably change the signal sent between a control center and the torpedo at short bursts within a range of 88 frequencies in the radio-frequency spectrum (there are 88 black and white keys on a piano keyboard). The specific code for the sequence of frequencies would be held identically by the controlling ship and in the torpedo. It would be practically impossible for the enemy to scan and jam all 88 frequencies, as this would require too much power or complexity. The frequency-hopping sequence was controlled by a player-piano mechanism, which Antheil had earlier used to score his Ballet Mécanique.[ci