"dB" stands for deciBells. A way to express a ratio. 1 bell = log (ratio) so the decible value ( deci meaning 1/10) is 10*log(ratio) where the ratio can be of two things, or of one thing to a standard. Radio and radar folks use dBW, where the "W" means relative to 1 Watt. "Z" is a meausre of reflectivity per unit volume. It has units of square meters per cubic meter (equivalently less instructive, 1/meters) In the case of rain, or snow, or chaff, it's a mesure of how much stuff is in that cubic meter of volume. In this case how much rain. But remember it a logrithmic scale so while 10 DbZ is 10 times more stuff than 0 db (log(1) = 0) 20 db is 100 times more. and 30 dB is 1000 times more. Hope that helps. Had to refresh all that a few years ago when I was modeling the AWACS radar for a simulator/trainer for the back end crew members.