A potentiometer is a variable resistor with an adjustable contact. Turning a knob or moving a slider changes the resistance and often changes the output voltage.
Where potentiometers are used
Potentiometers are used for volume knobs, brightness controls, calibration adjustments, sensor position feedback, and user input on electronics projects.
Potentiometer as a voltage divider
A potentiometer can create an adjustable voltage from a fixed supply. That voltage can be read by a microcontroller or used to control another circuit.
In simple terms, a potentiometer gives a circuit an adjustable setting.