input gain on most mixers allows control of the level of the input signal.
For optimum performance, this level should be set with the input and master
faders at 0 dB.
input devices will require different gain levels. A microphone, for example
requires higher gain than a line signal. For this reason there is often
either a seperate gain control for a mic signal or a button located near
the gain pot that will allow switching between the two. Alternatively,
there may also be a "pad" button for signals the input amplifier cannot
handle (most pads are in the area of -20 dBs).