Sidechaining is when a signal processor is controlled by a signal that’s different than the one it’s effecting. An example would be the compressor on a bass reacting to the kick drum. This is done in order to create a tone curve that would cause the compressor to act very specifically.
A compressor has two circuits that work in conjunction. One is the signal path itself which is the signal from input to attenuation to output. While the other is the detector circuit which controls the attenuator. Another name for the detector circuit is the “sidechain” because it’s a chain of components that happens on the side of the primary signal chain.
Read more about this here: https://theproaudiofiles.com/sidechain-compression/