Two resistors split a voltage. This is how you read resistive sensors (LDR, thermistor) and scale a higher voltage down to something an ADC pin can safely measure.
For a sensor, put the changing resistance as R1 or R2 and watch Vout move.
Output voltage
Vout = Vin × R2 / (R1 + R2) = 5 × 10kΩ / 20kΩ
50% of Vin
Current through divider
0.25 mA
Power burned
1.3 mW
Where you’ll use this
Reading an LDR or thermistor, scaling a 12 V battery down to the 3.3 V an ESP32 ADC can read, or setting a reference voltage. Keep total resistance in the kΩ range to avoid wasting current.