Instructables user Kurt E. Clother shows how to make a shunt regulator out of a Zener diode. A shunt regulator provides a path from the supply voltage to ground via a Zener diode, which reverses polarity when the voltage reaches a certain level, effectively regulating the voltage.
All diodes have a “reverse breakdown voltage” which if applied to the diode in reverse will cause current to flow backwards through the component, typically destroying it in the process. This value is typically in the hundreds to thousands of volts. A Zener diode (similar to an “Avalanche Diode”) is a special sub-class of diodes that will allow current to flow in the reverse direction if the applied voltage is above a certain level without damaging the component. Of course, there are limitations to the voltage level and/or current flow, but those are things that the design engineer must take into consideration.