what's the plenum?

 

When you have more than a one piece intake, it is the upper most half which the throttle body bolts to.

 

It doesn't matter if it's 2 piece or not. For engines, the intake plenum is a large(ish) volume from which the engine draws air. Air enters it via the throttle body and leaves via the intake runners to each cylinder of chamber. In the attached pic of a 20B the plenum chamber is the aluminium part on top with "3 Rotor" on it and the throttle body bolted to it. A typical rule of thumb is the plenum should be double the engine's capacity, but usually isn't for space reasons.