High pass means you have a filter that only allows midrange and high freequencies to be play...you can set the frequency at which you high pass and also the "slope". The filter isnt a total cutoff, it rolls the sound off from that frequency.
How you do it? Usually amps have crossovers built in, a lot of HU's do too now. Otherwise you can use a small capacitor as a simple high-pass filter
The image below is an example of what a crossover rolloff looks like (this is an example of high-pass):
EDIT: here's the pic:
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