Sidetone (also known as microphone monitoring) allows you to control the amount of mic input that is routed back into your headset, so you can choose how much of your own voice and surroundings to hear.
The adjustment of the sidetone for the Arctis 9 is done inside SteelSeries Engine software on a PC. The setting will be stored on the Arctis 9 for use with other devices via USB.