What is a throttle body and where it located on 2002 trailblazer?

The throttle body is a housing containing a valve (the throttle plate) that regulates the amount of air flowing into the engine. It's essentially the "gas pedal" for the engine, controlling the air/fuel mixture.

On a 2002 Chevrolet Trailblazer, the throttle body is located on the intake manifold. To be more precise, it's on the top of the engine, where the air intake tube connects to the engine. You'll find it at the front of the engine, near the air filter housing. It's a relatively large, cylindrical component with various sensors and vacuum lines connected to it. Look for a butterfly-shaped valve inside the housing (you may need a flashlight to see it clearly).