Voltage divider
A voltage divider is two resistors in series across a source, with the output tapped from the joint between them — it splits the input voltage into a smaller, precise fraction. Imagine a tall water tower and a tap halfway up its pipe: the pressure at the tap is a set fraction of the pressure at the bottom, fixed by the geometry. Here the 'geometry' is the ratio of the two resistances.
The output is V_out = V_in · R2/(R1+R2): the bottom resistor gets the same fraction of the voltage as it does of the total resistance. Dividers set bias points, scale sensor signals to fit an ADC, and create reference voltages. The crucial caveat is loading: if whatever you connect to the output draws appreciable current, it acts like a third resistor in parallel with R2 and pulls the output below the ideal value, so dividers work best feeding high-impedance inputs.
Rule of thumb: pick divider resistors so the current through them is at least ten times the current the load draws, keeping the loading error under about 10%.