Mic-Line Switch   

Mixer input channels may be provided with a switch to set input sensitivity to accept the low signal levels of a microphone or the relatively higher levels of a line signal (tape recorder, signal processor, synthesizer, etc.). 
Problems occur when there is a mismatch between the incoming signal level and the sensitivity of the input channel. 
Sending a line level signal to a channel with the mic-line switch set for mic input will result in a very distorted signal as the channel input will be overloaded.  Conversely, sending a microphone level signal to an input channel set for line level input will result in a weak and noisy signal at the input. 
Incoming signal levels and input channel sensitivity should agree as closely as possible in order to achieve optimum signal level at the channel input.
See: input attenuation