When an engine is on but there is no force being applied to it by the wheels and no pressure is applied to the accelerator pedal - i.e., the engine is turning at the lowest possible speed (measured in rpm) - it is said to be idling. The idle speed on a car is usually around 700-800rpm (the exact numbers can be found in your manual or (usually) written somewhere within the engine compartment, often on the underside of the hood), and is higher for cars with manual transmissions than automatics. The idle speed is adjusted by turning the idle speed adjusting screw.