Turn on the switch AFTER starting the simulation, else the simulation will not run
Switched Mode Power Supply. This is a simplified/crude version of what (usually) sits in your phone charger (and many other things). The lack of a transformer allows the circuit to be smaller. A transistor that rapidly switches between fully ON and fully OFF also reduces power loss as heat (which would be high if the voltage reduction came just from resistors)
The ~3 ohm resistor represents the load (such as your phone)
Edit: Circuit updated to be more stable and handle higher load as per recommendations of comments
