An RO solenoid valve is an electrically operated valve that controls the flow of water in a reverse osmosis system. It works by using an electromagnetic coil to open or close the valve when an electrical current is passed through it. This type of valve is crucial in controlling the flow of water through different stages of the filtration process, ensuring that water is filtered efficiently and without unnecessary waste. The solenoid valve acts as a critical control point, managing the timing of water entry and exit, and maintaining optimal pressure for the system.