Spool Valve is the metal piece inside control valves that slides to open or close oil paths. We push levers or send pilot oil to move the spool back and forth, and it aligns holes so oil goes to the right cylinder or motor. It has grooves cut into it so fluid flows only where we want. When it wears, oil leaks and movement gets sloppy. We replace or polish spools and check seals to keep our controls crisp. Think of it like a slide puzzle, but for oil flow.