What are plenum doors and where are they located in the heating system for a 1995 Ford Explorer?

The plenum door is inside your heat/AC system near the heater core. It opens and shuts incrementaly to allow the heater core to heat the air passing by, or closes to keep passing air from heating at all. If you open the glove box on a 95 explorer, you can see the plenum motor... and you should be able to see/hear it moving as you adjust the temperature control dial from cold to hot.