Drain Water Heat Recovery System - another fancy and long-winded title for a special plumbing fixture that helps you save on heating your hot water - a huge contributor to your utility bill (~30%).
It doesn't look fancy, and most of the time, it'll be hidden behind walls, but this little guy helps in big ways by attaching to your home's central drainage system and taking as much of the heat from the wastewater as possible. The captured heat from the pipe's coils is then transferred back into your water line, making things easier on your hot water heater.