Here are a couple pictures of throttle bodies. There are one or more round flat blades inside a bore. When they are sideways in the bore, they let very little air through; which makes the engine return to idle
When you press the accelerator pedal, you're rotating the shaft that those blades are on, turning them parallel to the airflow letting more air through. Hence the term, wide open throttle.
The throttle body is the whole casting around which these bores and blades are housed. In the first photo, disregard the blue arrow.
