A check valve is probably in the discharge line of the pump.
It allows flow in one direction and automatically prevents back flow (reverse flow) when fluid in the line reverses direction.
Valve Standard: Conforms to AWWA C508, Clear waterway
Working Pressure: 21 Bars/300PSI
Fusion bonded epoxy coated RAL3002
Flanged Ends: ANSI B16.1 Class 125/150/250, EN 1092-2 PN10/PN16
BS10 Table D/ Table E, Stainless Steel Stem