A plenum is basically a water space below the substrate with very little water circulation. The idea is that the water in there has little to no oxygen which is the environment in which denitrifying (?) bacteria thrive. These are the ones that trun nitrate (the end product of the nitrogen cycle in an oxygen rich envrinment) into nitrogen and other products. I've read mixed reviews from folks who have used them, and most people beleive you accomplish the same thing or better with just a deep sand bed. I've never tried one, so I can't speak from experience at their function.