Why A potentiometer is a variable resistor: it has a resistive element with a wiper that slides along it, so turning the shaft changes the resistance between the wiper and the ends. Volume controls and tone controls are common examples. Because changing resistance in a circuit changes the voltage division, a pot is often used to set a variable voltage, but the parameter it actually varies is resistance.
Watch out Variable inductance comes from a tuned coil or roller inductor, and variable capacitance from a variable capacitor such as the air-spaced type in older tuners; neither is what a pot adjusts.
Pot = potentiometer = variable resistor. The knob moves a wiper along a resistive strip.