A buck regulator is a type of switching regulator, a highly efficient means of producing a regulated DC voltage or current from a higher input voltage.
With the switch on, current flows through the inductor I1, charging the output capacitor C1. When the switch turns off, the diode D1 completes the circuit with the inductor, applying a voltage of the opposite polarity to the inductor, and causing the current to linearly decrease. A feedback loop adjusts the duty cycle or frequency of the switch SW1 in order to maintain a constant output voltage on the output capacitor.