Variable trimmer capacitors are types of capacitors(devices which store electrical charge in circuits) where the capacitor capacitance (its ability to store electrical charge) can be readily adjusted. They're often used in LC circuits (circuits that have an inductor and a capacitor) to set the frequency, for example to tune a radio.
Variable trimmer capacitors are used to initially set oscillator frequency values, rise and fall times and other variables in a circuit. Should the values drift over time, variable trimmer capacitors allow you to re-calibrate your equipment.
Variable trimmer capacitors are almost always mounted directly on a PCB (printed circuit board) and set by using a small screwdriver. Variable trimmer capacitors are usually cheaper than full-sized variable capacitors.
Common applications include oscillators, tuners, crystal oscillators and filters. Variable trimmer capacitors can also be found in communication equipment such as mobile radios and aerospace transmitters.