Find the cutoff frequency of a first-order RC low-pass or high-pass filter from the resistor and capacitor, or solve for a component to hit a target frequency.
The cutoff frequency where the signal is attenuated by 3dB is fc = 1 / (2π·R·C). The same RC sets a low-pass (output across C) or high-pass (output across R) filter; the time constant τ = R·C is how fast it responds. First-order rolloff is 6dB per octave.
It is the frequency where an RC filter has reduced the signal by 3dB (to about 70% of its amplitude), marking the boundary between the pass band and the stop band. It is fc = 1 / (2π·R·C).
Both use the same R and C and the same cutoff. If you take the output across the capacitor you get a low-pass (passes low frequencies); across the resistor you get a high-pass (passes high frequencies).
τ = R·C is the time the filter takes to respond, roughly how long the capacitor takes to charge to 63%. A larger time constant means a lower cutoff frequency and a slower response.