Check valve
A one-way valve that lets water flow in only one direction.
A check valve allows water to move one way and blocks it from flowing back. On a sump pump discharge line, it stops the water you just pumped out from draining back into the pit and making the pump cycle endlessly. A failed check valve makes a sump pump run more often and wear out faster; a faint 'thunk' when the pump stops is the valve doing its job.