IACV == Idle Air Control Valve. It is an emissions control device (basically) that slowly decreases the amount of air going to the engine to help burn off excess fuel when your throttle plate snaps closed (after you take your foot off the gas). It also controls the idle of your engine.
You should only replace it if it's broken. You will know when it's broken if it makes a 'moosing' sound like a foghorn, or possibly if you start your car and it tries to idle but dies immediately.
I believe the part is about 100 bucks from FordPartsOnline.
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