A diode is a two-terminal passive device that essentially acts as a one-way switch for current. Its function is to control the direction of current-flow. An ideal diode has infinite resistance in the backward direction, but zero resistance in the forward direction: thus, current passing through a diode can only go in one direction, called the forward direction. Current trying to flow the reverse direction is blocked.

In the above schematic, if power is pushed through pin 3, the active buzzer will work as the diode is forward-biased, but if power is pushed through pin 13, the active buzzer will not work as the diode is backward-biased.

• https://learn.sparkfun.com/tutorials/diodes/all