A common guitar tone control is a low-pass filter. It's made from pickup resistance and tone capacitor capacitance. It's called also a RC filter.
A tone pot in a guitar is used as a variable resistor (only two lugs are used). It's placed after or before the tone cap. The more resistance is on this path, the smaller amount of treble will bleed out to the ground. The diagram shows how it work's in three different knob positions:
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