Two resistors in series across a voltage split it between them. The voltage at the midpoint is a fixed fraction of the input, set by the ratio of the two resistances. This is how a field device scales a battery higher than 3.3 volts down to a level its logic can safely read. Drag the input voltage and the two resistors and watch the midpoint follow Vout = Vin × R2 / (R1 + R2).
Vout = Vin × R2 ÷ (R1 + R2) = 2.50 V
The fraction depends only on the ratio of the resistors, not their size. 10 kΩ over 10 kΩ keeps half; a larger R2 keeps more of the input, a larger R1 keeps less. The input voltage scales the result but never changes the fraction. To halve a 12 volt input down to a safe 6 volts, you make R1 and R2 equal; to bring it to one third, you make R1 twice R2.