Sidechain compression can be a powerful tool for both making space in a mix, and creating rhythmical relationships between instruments.
What do I mean by Sidechaining.
In audio, sidechaining is using one signal to affect another. The most common example of this is with compressors. Compressors effect the dynamics of audio and we can use another signal to tell the compressor exactly how to effect the dynamics of the signal.
This idea came from film audio engineer, Douglas Shearer in the 1930s. He needed a way to control the sibilance ("sss" sound) in dialog. So he created a compressor with a "side" signal chain, separate from the main trigger signal, attached to an EQ. The result was the compressor would only engage when the harsh "S" sound was heard, instead of evenly compressing the full signal. The resulting technique was Sidechaining.
Now, we can hear it in all kinds of genres, most commonly in Electronic Dance Music (EDM) but also in others such as pop and Contemporary Christian Music. When you hear the music "pulse" with the Kick drum, your probably hearing Sidechaining. This can help the music build and create some interesting rhythmical relationships. As well as this, it can also make space for other instruments as it ducks the volume of one audio signal as another audio signal is played.
This is achieved through compressors. With a compressor as an insert for a sound, another signal is used to activate the gain reduction of the compressor, bringing the volume down when 'activated' and the release settings allow the signal to return to the original level.
The same effect can also be achieved through the use of Low Frequency Oscillators (LFOs) within synths or through he use of plugins.
If you are a reason user Synchronous is a great too for achieving this: https://www.reasonstudios.com/shop/rack-extension/synchronous/
Alternatively LFO tool by xfer records is perfect for most DAWs: https://xferrecords.com/products/lfo-tool
See how this is done in practice in the video below