Sidechaining is a production technique used in a wide variety of music genres where the volume of one track, influences another track.

A most recognizable example is sidechaining a kick (as input) and some synth part or a bassline, so that the latter is muted when the kick plays. This creates a rythmic "pumping" effect. A more subtle use of sidechaining, is using to help make specific tracks of your song cut through the mix.

You can also use sidechaining to temporarily attenuate ("duck") effects like reverb and delay - again to help make specific tracks of your song cut through the mix. You can also use this as a special effect.