Sidechaining, explained: "Even if you’ve not used it directly in your music-making, you’ll probably have come across it both in the studio and in your daily life"

Alesis 3630.

Audio equipment such as gates and compressors will typically feature two signal paths: one being the signal that’s being processed, and the other the signal that actually controls the behaviour. 

The control signal is usually generated by monitoring the device’s input level and is known as a sidechain – however, a lot of processors also offer the option of feeding in an external signal to control the processor’s behaviour.