Wet sanding is using a liquid to help remove some of the excess material and also keep the scratching down. It can be but does not have to be water. Water is used in similar processes such as sanding metal or sharpening cutlery on a wet stone. There are many types of oil that can be used when finishing wood surfaces. Linseed and tung oils for example. It depends on the final finish that you want on the project.....Mark Hammond

Quick tip. Use 2000 grit wet/dry sandpaper and a spray bottle filled with water and dish soap. mist the surface and lightly sand in a circular motion, you should fell that is moves real smooth over the surface with no drag, if you start to feel drag, stop and wet some more. Keep doing this until you cover the entire surface. Wipe it clean and let it dry. When you are done you should have a smooth satin finish, if you want more gloss, depending on what finish you used, you can buff it with car polish, or use pmice or rotton stone and a felt pad to obtain a higher gloss.