A resistor is commonly made of carbon if being used for low wattage. Resistors reduce electrical pressure (voltage) and flow (current). These fixed resistors are very uncommon for unpowered guitar circuits, because there is no internal amplifier to bias. Variable resistors (potentiometers) are resistors that change resistance when you turn them, this is wiping a conductor across a phenolic wafer either closer to ground (more resistance) or closer to the output lug (less resistance). Values affect potentiometers ranges of resistance. If a potentiometer is coupled with a unipolar capacitor connected across one output and common (ground), it can cut high (it's a low pass filter [LPF]) frequencies at a variable amount. There is a breif explaination of resistors, pots, and how they apply to a guitar's tone cirucit.