Check valve allows flow in one direction and automatically prevents back flow (reverse flow) when fluid in the line reverses direction. They are one of the few self-automated valves that do not require assistance to open and close. Unlike other valves, they continue to work even if the plant facility loses air, electricity, or the human being that might manually cycle them.
A check valve is probably in the discharge line of the pump. Outside the home, they are found in virtually every industry where a pump is located. UL & FM certified.