a Zener diode
/ ZEE-ner /
An ordinary diode is something you keep OUT of reverse breakdown. A Zener diode is built to live there. Run it backwards and, once you reach its rated breakdown voltage, it suddenly conducts and then clamps — holding a nearly constant voltage across itself almost no matter how much current flows. That makes it a crude, cheap voltage reference: a fixed voltage 'wall' to lean other circuits against.
You use it as a shunt regulator: put a series resistor from your messy supply to the Zener, and the Zener across the load. The resistor drops the excess; the Zener soaks up whatever current is left over to hold its node at, say, 5.1 V. If the load takes more current, the Zener gives up some of its share; if less, it takes more — always keeping the voltage parked at its breakdown value. A 5.1 V Zener with a resistor sized to pass a few mA gives you a steady-ish 5.1 V rail.
Honesty: it is crude. The voltage drifts with current and temperature, it is noisy, and the resistor wastes power continuously whether or not the load needs it. For a rough reference, a clamp, or a protection limiter it is perfect; for a precise, low-noise reference you would use a bandgap reference instead. (Above ~5 to 6 V the mechanism is actually avalanche, not the true Zener effect, but the part is still sold as a 'Zener'.)
A 12 V supply, a 470 ohm series resistor, and a 5.1 V Zener: the resistor drops 12 - 5.1 = 6.9 V, passing 6.9 / 470 = ~15 mA. The Zener and load share that 15 mA while the node stays near 5.1 V.
A Zener plus a series resistor is a simple shunt regulator / reference.
A Zener is a sloppy reference: its voltage shifts with current (its 'Zener impedance') and temperature, and the shunt resistor wastes power all the time. Use it for clamping and rough rails; use a bandgap reference when accuracy matters.