Sanding is something you do on wood that has already been treated, and therefore already stripped. It is a way of removing every last imperfection or residue that may be left on the wood. A first, vigorous sanding can be done with a suitable tool, with quite coarse sandpaper, so as to remove any residual material and any smudges or smears that show.
You should try to pass the tool over the entire surface of the wood, while maintaining even pressure for each area, to avoid unevenness. After sanding with a tool, use a fine grain sandpaper to finish the wood manually. When sanding by hand, try again to press evenly on the wood, but this time not to press too hard.