Each channel has its own fader (slider) to adjust the volume of the channel's signal before it is sent to the next stage (subgroup or master mix).
A slider is a potentiometer, or variable resistor. This is a simple control which varies the amount of resistance and therefore the signal level. If you are able to look into the inside of your console you will see exactly how simple a fader is.
As a rule it is desirable to run the fader around the 0dB mark for optimum sound quality, although this will obviously vary a lot.
Remember that there are two ways to adjust a channel's level: The input gain and the output fader. Make sure the input gain provides a strong signal level to the channel without clipping and leave it at that level — use the fader for ongoing adjustments.
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