A grindstone is a device for sharpening metal tools. If you're a farmer, or an ax-wielding goblin in a fantasy novel, you might be in need of a grindstone.
Classic grindstones are wheels made of stone that hone the edges of metal blades held against them as they rotate. You might use such a grindstone to sharpen your shovel or pruning shears. If someone tells you to "keep your nose to the grindstone," they basically mean "work hard!" This colloquial phrase comes from the old-fashioned practice of knife grinders keeping their faces extremely close to the grindstone as they worked.