Solenoid valve is a valve that is controlled by either AC or DC electric current through the coil / selenoida. Solenoid valve is the most frequently used control element in fluid systems. As in pneumatic systems, hydraulic systems or machine control systems that require automatic control elements. For example in a pneumatic system, the solenoid valve's duty is to control the pressurized air passage to the pneumatic actuator (cylinder). Or in a water reservoir that requires a solenoid valve as a water filling regulator, so that the reservoir does not empty. And various other examples that I may not explain one by one here.