Question

what is a zener diode?

Answer 1

A zener diode is a special kind of diode which allows current to flow in the forward direction in the same manner as an ideal diode, but will also permit it to flow in the reverse direction when the voltage is above a certain value known as the breakdown voltage, "zener knee voltage" or "zener voltage."

Answer 2

Let's add a few things: you can use zener diode to build a circuit that limits a signal within a prefixed range. That's because when the zener works above its zener voltage it acts as a voltage limiter