A downstem is the piece of a water pipe that connects the bowl to the main chamber, and in this case it is fixed to the pipe, but others can be removable. The downstem helps provide suction, pulling smoke or vapor into the water before getting inhaled. Downstems can have a diffuser at the chamber end, which additionally cools smoke.
“This heady glass piece I got has a fixed downstem that can’t be removed.”