Relays are electromagnetic switches that allow a low-power electrical signal (like a dashboard switch) to control a high-power circuit (like headlights or the fuel pump). For example, when you turn on your headlights, the switch sends a tiny signal to the relay coil, which magnetically closes a heavy-duty contact to send power directly from the battery to the lights. This prevents high voltage from running through the delicate dashboard wiring. A relay set typically includes standard 4-pin or 5-pin relays used for the horn, starter, fans, and lighting systems.