A backflow preventer is a device or assembly used to protect potable (drinkable) water supplies from contamination due to the reversal of flow in a water distribution system. 

It ensures that water flows in one direction only, preventing the backward flow of potentially contaminated water into the clean water supply.

 Backflow can occur due to a variety of situations, such as a drop in water pressure in the supply line.

Backflow preventers are typically required by local building codes and regulations, and they need to be installed, tested, and maintained by certified professionals to ensure they work effectively.