What these filters do in a pedal
A low-pass filter lets the lows through and rolls off the highs. In the schematic on the left, the signal goes through R1 and then C1 gives the high frequencies an easy path to ground, so they never make it to the output. This is the guts of almost every simple tone control: turn the knob, move the cutoff, and the pedal gets darker or brighter. Big Muff style tone stacks, one-knob tone controls, and the treble-taming caps sitting across a clipping stage are all doing this same job.
A high-pass filter is the mirror image: it lets the highs through and rolls off the lows. On the right, the signal has to get through C2 first, and a capacitor blocks low frequencies more than high ones, while R2 gives the output its reference to ground. You see this everywhere as coupling caps between stages (which decide how much bass survives into the next stage) and in treble bleed networks, which let the highs sneak past a volume control so your tone stays clear when you roll the volume down.
The cutoff frequency fc = 1 / (2πRC) is the point where the filter is down 3 dB, which works out to about 70.7% of the original signal voltage. It is not a brick wall. Past the cutoff, a first-order filter fades the signal out gradually at 6 dB per octave, so the musical effect starts well before the cutoff number and keeps going well past it. That is why a coupling cap that "cuts bass below 100 Hz" still thins out your low E string a little, and why small cap changes you can barely measure on paper are often changes you can clearly hear.