Explanation of heavily doping of both p and n sides of Zener diode
Circuit diagram of Zener diode as a dc voltage regulator
Explanation of the use of Zener diode as a dc voltage regulator.

By heavily doping both p and n sides of the junction, depletion region formed is very thin, i.e.
< 10−6m . Hence, electric field, across the junction is very high (even for a small reverse bias. This can lead to a 'breakdown' during reverse biasing.

If the input voltage increases/decreases, current through resister Rs, and Zener diode, also increases/decreases. This increases/decreases the voltage drop across Rs without any change in voltage across the Zener diode.
This is because, in the breakdown region, Zener voltage remains constant even though the current through the Zener diode changes.