What is side-chain in audio recording?

Sidechaining is where you have two signal, A and B and the signal B affects how signal A is processed. For example if you have a kick drum and bass track, if you put a compressor on the bass track and send the kick drum to the side-chain of that compressor, the compressor will act by how the kick drum plays, and thus gives more room for the kick to cut thru the mix as the bass ducks when the kick drum hits.