Zener Diodes
Normal diodes let current flow with the arrow. But the Zener diode wants to be different.
Zeners are designed to let current flow against the arrow. In the process, there is a consistent and very useful voltage drop across the Zener diode.
We can use the Zener to give us a specific voltage whenever we need one. This is especially useful when a circuit requires an exact voltage in, but we're using a power supply that may be inconsistent, like batteries that drain over time.
Zener's come in a range of values, including the handy 5.1 V Zener.