Here is sandpaper grit size in a nutshell. Sandpaper comes in all grits and sizes. Sandpaper sheets are covered in bumpy material that helps smooth or roughen hard surfaces or to remove a surface coating. This bumpy material is known as grit. The smaller the grit size, the coarser the sandpaper. Less grit means more space between the abrasive material, so it feel more rugged.
If you’re planning to quickly remove an outer coating, a small grit size is probably best because the coarse sandpaper will act as sharp teeth and more easily tear away the layer. If you’re doing a final sand of a nice finish, a large grit size is best because the subtle abrasive material will provide a very smooth finish.