Gain controls how much signal enters the mixer from the microphone or instrument.
To set gain:
Select the input channel.
Have the performer speak or play at normal performance volume.
Find the Gain control on the screen or processing section. (top left corner)
Slowly increase the gain and volume.
Watch the meter.
Set the level so it is strong but does not hit the red/clipping area.
If the gain is too high:
The sound becomes distorted.
The signal may clip.
If the gain is too low:
The sound will be quiet.
Noise may become noticeable.