A check valve is a mechanical device that allows fluid to flow in one direction while preventing backflow. It is commonly used in various applications, such as plumbing, water supply systems, and industrial processes, to ensure that liquids or gases move in the intended direction. These valves operate automatically, relying on the pressure of the fluid to open and close. When the flow is in the correct direction, the valve opens, allowing passage. If the flow reverses, the valve closes, preventing any backflow that could cause damage or contamination in systems like pumps and pipelines.