The 3 Series<Model> throttle body is a part of the engine's intake system which regulates the amount of air that is mixed with fuel for combustion.
The 3 Series<Model> throttle body houses a valve or flap know as the throttle plate which opens and closes to regulate airflow into the enginer. This is controlled by movement of the accelerator pedal by the driver although may also have additional input from the engine control unit (ECU).
The 3 Series<Model> throttle body is located on the top of the engine between the airbox/air filter and inlet/intake manifold which are often connected together with rubber, silicone or metal tubing.
Problems can occur with the 3 Series<Model> throttle body due to a build of of dirt or soot causing restrictions in the air flow or movement of the throttle plate.