The mic preamp (preamplifer) is where the input signal is conditioned so that it is suitable for further processing in the console.
The high-pass filter cuts low frequencies, generally below 100 Hz or so.
It is common for a microphone to receive excessive and unwanted low frequency signals. An example of this is a mic stand on a stage. Foot noise on the stage travels up the stand and enters the mic. If the mic stand is kicked, then this can create an enormous amount of LF energy.
Not only does this sound bad, it can potentially overload the mic preamp, causing distortion.
Ideally the filter should be placed before the active components of the preamp so that it cuts the signal before distortion can occur.
The filter is also useful in cleaning up the mix. Often it is adviseable to cut very low frequencies on every channel, apart from those dedicated to bass instruments.