If you want to properly calibrate your multichannel apps to match with your audio hardware, you have to provide a reference signal of a known level on each channel. You should then set the reference, for example -18 dB, with the reference slider and the app will do all the magic for you if you press the “calibrate” button below. If the level difference between the app and your hardware is much too high the slider range will not suffice. In that case increase the level/gain of your hardware.
You can also adjust the calibration sliders manually, although that is not recommended. If you are using the internal microphone of your device, calibration is not needed and should be reset to default.