Plenum is a word for all the 'tubes' within the manifolds, rough definition would be a an area for air movement, which is what an intake manifold does.
Chris, the spacer you are talking about is in fact just a spacer for the throttlebody. You would be using it to elongate the length of the runners of the intake for more airflow at some specific RPM the designer of the system decided on, also known as 'port tuning'. Toyotas and a few, well many now, other manufacturers run dual intake runners with butterfly valves that open at different times to increase either high RPM or Low RPM performance. (I hope I got that right, it has been awhile since I had to think about design specifics)
