A piston valve is a device used to regulate fluid flow in machines like steam engines and compressors. It has a cylindrical piston that slides within a chamber, opening and closing ports as it moves. This controls the entry and exit of fluids, ensuring proper timing, direction, and pressure. Piston valves are valued for durability, efficient sealing, minimal leakage, and smooth operation, making them suitable for high-pressure and high-temperature applications.