high pass or low pass are typically characterised by their cut off frequency. It's common to give the frequency at which the signal is damped 3db compared to the highest/unfiltered part of signal.
I want a pedal where i can change that frequency so i might just use a filter which cuts frequencies under 20Hz, which i don't hear anyway, or cut most lows around and under 85 Hz which is the resonance frequenz of a lot of speakers and the lows can get overbearing easily with neck PUs.
Or maybe i want to cut even higher, but i want pedal where i can set that frequency (and therefore variable Highpass)