Mixer Input Gain   

  The 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. 

Different 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).