10k rotary potentiometer with black-yellow knob - fits breadboard - pack of 3

Potentiometers are useful for setting or adjusting the voltage of a circuit. They can be used as controls. This one has a nut and washer that enables it to be panel mounted, but it also has the correct pitch to fit in a breadboard. Usage example...

Use the middle leg as the output. Connect 3V3 to one of the outer legs and GND to the other outer leg. When you rotate the knob, you will be able to produce any voltage between 0 and 3.3V at the output.

Coupled with an analog to digital converter or microcontroller, these devices can give you a very fine degree of control.