Why In frequency hopping spread spectrum (FHSS), the transmitter jumps its carrier from channel to channel many times per second following a pseudorandom hop pattern, and the receiver follows the same pattern in step. Because the sequence is pseudorandom, both ends can generate it from a shared key, so the signal looks like noise to anyone not synchronized. Spreading the energy over many channels also means narrowband interference only corrupts a few hops, which coding can recover.
Watch out The description of a binary bit stream rapidly shifting the carrier's phase is direct sequence spread spectrum (DSSS), the other main spread spectrum technique in the pool. A receiver telling the transmitter to change frequency is just adaptive channel selection, not spread spectrum, and clipping to make harmonics is a keying defect, not error correction.
Hopping = frequency changes; Direct sequence = phase changes. Both driven by a pseudorandom code.