Run your fingers across a board scrap, the bottom side of a practice drawer, etc. to familiarize yourself with the surface before sanding. Go a few strokes with coarse paper, a few strokes with medium paper, and a few strokes with fine paper in different areas. There is a noticeable difference in finish. Some parts will feel open and scratched, some parts will feel fairly smooth, and others will feel more smooth but not finished. That is a quick way to teach someone: Sandpaper grit is not just a number.
Grit is essentially how rough or fine the sandpaper is. Low grit numbers remove material quickly and high grit numbers remove material slowly. On furniture, you want to be very selective about which parts you use your sandpaper grit on. If you want to clean off a small table top, chair rail, or drawer front, first determine what condition the wood is in to decide what kind of sandpaper grit to use. Is the finish on the table dirty, waxy, or oily? Coarse sandpaper will not be good to use here as the grit will get clogged up. Is the furniture a thin veneer? Using coarse sandpaper here will likely take off more wood than you intend to.
For most furniture sanding projects for beginners or small projects that require minor sanding, I would start with a medium sandpaper grit. A medium sandpaper grit would do well to take off or dull the top surface, get off very light scratches, or get off top layers of paint without getting off the detail or edges of the board. I would also not use coarse sandpaper in every spot on the board where I have a ding, water ring, or chipped edge. A coarse grit will only be used after inspecting the furniture board and deciding a particular piece of furniture is in really rough shape, maybe after a finish job has failed, or for sanding off a rough patch of bare wood in a good condition piece that does not need refinishing.
It is just as important to sand in a good technique with your hand and movement as to select the correct sandpaper grit. Use a sanding block and sand in the direction of the grain. This will prevent you from gouging out areas. You want to avoid putting grooves and marks in the surface of your board. On edges, details, and corners, you want to reduce the amount of pressure you apply, and sanding lightly on these areas can be very important. A rounded table edge or detail can be changed or rounded off quite quickly during sanding and may not look right once finished.
You will need to go back to a fine sanding grit to get your surface smooth. It is a good idea to clean the surface between changes in sandpaper grit. If you are using a medium sandpaper grit to get out deep sanding scratches on a surface, take your time to go to a fine grit to avoid putting new, even deeper scratches in the surface. You do not want to put stain on a board that you can see and feel still has sanding scratches in it. This only works until your finish starts to yellow and you start getting more dirt and grime that will catch in the deep scratches. Take a soft clean cloth, tack cloth or even your hand to clean your board to see if any sanding scratches are visible to you.
New to sanding furniture may stay on a fine sandpaper grit for longer than they need to. I see fine sandpaper grit on the back of sandpaper papers as a surface smoother. I will not use fine sandpaper grit for deep scratches. If you have chipped and filled your wood filler to a small ding, make sure that filler is sanded down until it is level with the rest of the board first before using a finer sanding grit. If you have deep sanding scratches, go back to a coarser sanding grit until you get a smooth board surface and then sand with the finer sanding grit. This is a much better use of your time when compared to just sanding a small ding with a fine sanding grit and still feeling the scratch.
Before putting on your stain, paint, wax, oil finish, or topcoat, make a small sample in a place you do not see. I have made it into a habit to use a piece of scrap wood or a small hidden section to put my sample stain or finish before applying stain, paint, or finish to the board. Use the same sandpaper grit sequence as the furniture you are sanding. Look at your sample and decide if you like the color, or finish. Do you like the way the sanding scratch looks? Can you see brush marks? Is your board surface cloudy or too glossy or shiny? It is a good idea to sand your furniture pieces with the best sandpaper grit possible that you can get to get the furniture surface ready without over-sanding. Your furniture pieces should still keep its character and you want to make sure that the furniture surface is ready to put on your stain or topcoat finish.