The throttle body is the housing that contains the Throttle Plate, Idle Air Control Valve, and Map Sensor. The throttle cable is also connected to this. The throttle body essentially limits how much air is allowed to enter the engine, thereby regulating it's speed.

When fully opened, so that the Throttle Plate is parallel with the intake air stream, the area of the open trottle body becomes the limiting factor on how much air the engine can take in. This isn't usually a problem unless you have a highly modified race engine. In these applications, a larger throttle body ''MAY'' open up the possiblity for more top-end power.