The Idle Air Control Valve (IACV) controls the engine's idle speed. When the engine is idling (not accelerating), the IACV regulates the amount of air entering the engine to maintain a consistent RPM. It does this by controlling a small passageway that allows air to bypass the throttle plate.
In simpler terms: The throttle plate is mostly closed at idle. The IACV opens or closes to let just the right amount of extra air in so the engine doesn't stall or run too fast when nothing else is demanding air from the engine (like the accelerator pedal).
