A potential divider is a simple circuit with resistors arranged in series to obtain a fraction of the main voltage provided by a voltage supply. The voltage is divided between the resistors; the greater the value of the resistor, the bigger the proportion of the voltage it will take.
Two Fixed Resistors
Variable Resistor / Rheostat
We can change R1 or R2 to be a variable resistor so that we can easily vary Vout without replacing the resistors.
A potentiometer is a variable resistor that is connected at three terminals shown as points A, B, C. Contact C is a sliding contact.
The position of C determines the ratio of resistance of AC to BC since resistance is proportional to length (R = ρL / A) for a fixed cross-sectional area.