Scott, the "grade" (or grit) of abrasive paper (sandpaper) refers to the size of the abrasive particles in the sandpaper. Given the same number of passes and the same amount of pressure paper with larger particles sands deeper (and rougher) than paper containing smaller particles. A lower number indicates that the grade of the paper is used for rough sanding where a high number indicates the sandpaper is meant for finish sanding. Generally speaking 30- and 60-grit papers are used for rough sanding, 100- to 150-grit sandpaper is for medium sanding and 220-grit sandpaper is used for finish sanding. Of course, this changes with the type of wood and whether the sanding is done by hand or with a machine. Sanding a soft wood with rough sandpaper could possibly tear the wood fibers (the grain). Sanding perpendicular to the wood fibers also could tear the wood fibers. When the fibers tear they raise from the surface. Another way of causing the "grain to raise" is to over-wet wood. The best way to determine what grit to use is to test-sand. Keep in mind that rough grits of sandpaper leave deep scratches so start with the finer grits (150 to 220) and slowly work up to the rougher grades. Raising the grain is what painters must contend with after the first coat of paint is applied. At this point, and once the paint or varnish has dried, the first coat and the raised surface must be smoothed. The second coat of finish usually will not raise the grain. This is because the wood is protected from absorbing moisture by the previous coat.