You may have heard of the term, "side chain" or "sidechain" in audio recording. As with so many things in the audio recording realm, the term sounds more complex than what it actually means.
Okay, let's start with a basic description of how an effect (think compression or EQ) is applied to an audio track.
Let's say you want a compressor (see my post "Should You Use Compression In Audio Recording?") on your musical instrument track. You would connect the compressor by sending the music on a round trip from the music track to the compressor. When the music comes back FROM the compressor, it is now compressed. See pic below.
A compressor only applies its effect when it is told to by the incoming audio. For a review on this see my post Vocal Compression Using Reaper's ReaComp Effect Plugin. For example, it will compress only if and when the incoming audio gets louder than a certain "threshold" volume. At all other times, the compressor does nothing.
Now usually the sending and receiving signals are from the same track - instruments from the instrument track go into the compressor and compressed instruments go back to the instrument track.
Say you could get the compressor to compress the vocal track based on instructions from a track that is NOT the vocal? whoa, that's some mad scientist stuff right there! But maybe you could do some cool things.
Let's say you have music on a second track. What if you sent THAT audio into the compressor to control how the voice is compressed? Well. that is exactly what happens when you "duck" an audio signal. When the voice is singing/talking, it pushes the music out of the way. You guessed it. I have a post on ducking too:) - What is Ducking In Audio Recording?
You still have the instrument track sending the music on a round-trip to the compressor. But the compressor no longer takes orders from the instruments. It listens to a new master whose input comes in a side door:). Hence the moniker "side chain."
That side door is where you send the signal from the vocal track. So the instruments still get compressed (turned down when the control signal gets loud enough), but it's now the vocal volume acting as the control. So when the vocal goes above the threshold, the compressor turns the volume down on the instruments. Pretty cool huh?
Hopefully this gives you an idea what sidechain (side-chain, or however you spell it) in audio recording means.