Quite simply a stepper motor is a motor that moves in discrete steps.  There
are various sorts.  In the type of use you are referring to you can just
think of it a a servo motor that provides linear control/motion.

Couple such a motor to a mechanism for controlling the amount of air that
can be used by the engine under closed throttle conditions and you have
yourself an Idle Control Valve.

Does this help?

Nick /////