A “9-inch grinder” means the tool uses a disc (wheel) of around 9 inches (≈ 230 mm) diameter.
It is a type of Angle Grinder — a hand-held power tool whose motor drives a gear set so the disc spins at high speed at a right-angle orientation.
Compared with smaller grinders (4.5", 5", 6"), a 9′′ unit is categorized among the “large/ heavy-duty” grinders.
Because of the large disc size, the tool has a large cutting/grinding depth and is built for high material-removal tasks: cutting thick metal plates, steel beams, masonry blocks, concrete slabs, large rebar, etc.