The mic preamp (preamplifer) is where the input signal is conditioned so that it is suitable for further processing in the console.
Each channel of the console has two input sockets; mic input and line input.
The mic input can accept the output of a microphone (naturally) or a DI (direct inject) box. Nothing else should be connected to a mic input.
Anything else will be connected to the line input.
A microphone has a low output level. A typical figure would be 10 mV (one hundredth of a volt). The console likes to work on a signal level of around one volt to keep well above the inevitable noise signals that will be present.
This means that the signal has to be boosted by 100 times, or 40 dB (40 decibels).
Of course, the level that comes from the microphone depends on the level of the sound source, and its distance. In practice a mic preamp needs a range of gain of 20 dB up to 60 dB. Some preamps go beyond this range, from 0 dB to 80 dB. 0 dB means x1 or unity gain - in other words no gain at all. 80 dB means x10,000. This would cover all situations from a mic being placed 10 mm from the point where the kick drum pedal hits, to a watch ticking at twenty paces.
There would be no advantage in providing more than 80 dB of gain because the noise produced by the mic would be amplified above the console's noise level.