This circuit is very useful in beginners circuits, since most will work on 5v, which is the voltage of this easy little circuit.
How it works:
This circuit is just an implementation of the 7805 integrated voltage
regulator. What this little component does is to lower a voltage and
stabilize it by reducing noise and ripple, in order for circuits to have
the constant voltage needed to work correctly
You can find the datasheet by following the link: 7805, take note of the typical applications notes, you can find some more uses for this versatile little regulator