What electrical parameter is controlled by a potentiometer?

A potentiometer is fundamentally a variable resistor. When you turn its shaft or slide its wiper, you physically change the amount of resistive material the current must travel through, thereby altering its resistance value. Because it is constructed entirely as a resistive device, it has no effect on inductance, capacitance, or magnetic field strength. Those specific electrical parameters are controlled by variable inductors, variable capacitors, and magnetic devices, respectively.