A check valve is a one-way valve that lets water flow in a single direction and closes automatically to block it from flowing back. It works on pressure alone, with no handle. Plumbing code requires one on sump pump and sewage ejector discharge lines to stop pumped water from draining back into the pit.
A check valve is a valve that allows water to move in one direction only. When water flows the correct way, the valve opens and lets it pass. When the flow slows, stops, or tries to reverse, the valve snaps shut and blocks it. There is no handle and no one to operate it. The valve reacts to the water itself, which is why it is also called a one-way valve or a non-return valve.
The mechanism is simple and driven by pressure. Inside the valve is a moving part, usually a hinged flap (a swing check) or a spring-loaded disc (a spring check). Forward flow pushes that part open. When forward pressure drops away, gravity, the spring, or the weight of backward-pushing water forces the part back onto its seat, sealing the pipe. A swing check valve works best in horizontal pipe, while a spring check valve can sit in either a horizontal or a vertical line.
The most common place plumbing code requires a check valve is on a sump pump or sewage ejector discharge line. These pumps sit in a pit below the drain level and push water up and out. The moment the pump shuts off, all the water still standing in that vertical discharge pipe wants to fall straight back down into the pit. For example, without a check valve a basement sewage ejector would drain its own discharge pipe back into the sump every cycle, so the pump would run constantly, wear out fast, and never keep up. The check valve holds that column of water in the pipe so it cannot flow back.
A check valve is related to, but simpler than, a backwater valve. A backwater valve is a heavier one-way valve built into a building's main sewer line to stop a city sewer backup from pushing sewage into the house. Both block reverse flow, but a check valve is the general one-way device used throughout a system, while a backwater valve is the specific sewer-protection version. A worn check valve that no longer seals is a common reason a sump pump short-cycles or a line hammers when a pump stops.