The solenoid valve of a humidifier is found in the water supply line and is the component that controls the flow of water to the main body of the humidifier. "Solenoid" is a generic term for a device or component that uses energy to create linear motion.
The solenoid valve is controlled by the humidistat, the component of the humidifier where the user sets the desired level of humidity in the air. Depending on the level of ambient humidity relative to its set point, the humidistat either completes or breaks an electrical circuit to the solenoid, opening or closing the valve in the supply line. This process controls the flow of water to the main section of the humidifier.