In a commercial building the plenum is the space between the dropped ceiling and the structural ceiling or floor above. It can contain lighting housings, wiring or cables, HVAC ducting, and sprinkler systems.
On the NCIDQ Exam, you'll need to understand this for all 3 exams. You'll coordinate with other building systems on PRAC 2.0 to ensure that there is adequate space to meet the design intent. You'll annotate the drawing and schedules for consultants, like MEP, and show how you would possible changes and solutions to any conflicts.