Why A spread spectrum receiver multiplies the incoming signal by the same spreading code (frequency hopping sequence or direct sequence chip pattern) that the transmitter used. That correlation process collapses the wanted signal back to its narrow bandwidth while any signal that does not follow the same code gets smeared across the wide bandwidth, dropping its power density far below the recovered signal. The result is strong rejection of interference without needing high power or feedback to the transmitter.
Watch out High transmit power is not the mechanism, and in fact Part 97 limits amateur spread spectrum power to 10 watts; error correction and adaptive frequency changing are separate techniques that some systems add, but they are not what makes spread spectrum inherently interference resistant.
Only the matching code gets de-spread; everything else stays spread out and fades into the noise.