The fuel pump relay is an electromagnetic switch that controls the power supply to the fuel pump. When you turn the ignition key, the ECU triggers this relay to prime the fuel system for a few seconds. Once the engine starts, the relay stays closed to provide continuous power. If the relay fails (a common issue in older cars), the fuel pump will not run, and the engine will crank but never start. It is a safety feature as well; if the engine stalls in an accident, the relay cuts power to the pump to stop fuel flow.