A simple voltage divider can "step-down" a voltage. For example:
simulate this circuit – Schematic created using CircuitLab
However, the 5V output of the circuit above is unregulated. This means that
(1) the output voltage will vary in proportion to the input voltage
(2) the output voltage will vary with output current
Now, imagine that R1 in the schematic above were variable and controlled in such a way that, within a certain range of input voltage and output current, the output would be 5V regardless.
A simple implementation of such an idea is:
The transistor and base bias circuit form a dynamic voltage divider with the load resistance, "dropping" the input voltage down to the desired output voltage and maintaining that voltage relatively constant with respect to variations in input voltage and output current.