When you paint with oils, the surface doesn't naturally come out flat. There are normally little bumps and streaks formed by application and manipulation of the paint. Some artists deliberately use this tendency to create a textural or sculptural effect; this is called impasto. However, in multilayered painting, surface texture is often not desired. If you are going to add further layers, you often want to add them to a flat surface. This is especially true with glazing, since texture will create places where extra paint pools, creating a mottled effect. You can reduce surface texture in wet paint by gently feathering with a clean dry brush after you're done painting. That tends to blur edges slightly, however, which may not be desired. And sometimes you miss a spot.
Once the paint has dried thoroughly, it is sometimes advantageous to sand the surface down slightly. In order sand very smoothly, and in order to avoid breathing pigment dust, it is best to use a wet sanding technique. Lay the painting down on a flat surface. Spray some water on the surface, or wipe it down with a wet cloth (adding a few drops of dish detergent is helpful for lubrication). Now use a wet green kitchen scrubee pad to gently sand the surface. Unless you want to remove some mistake, the idea is to lightly rub the pad around, without applying significant downward pressure. Some paint will come up and the water will change color; that's OK. Do this over the whole surface until it feels smooth. Now, before any evaporation occurs, wipe all of the dirty water off of the surface with a paper towel. That way, you don't have to worry about pigment dust. Let the painting dry before painting on it again; often an additional day is a good idea to allow any tacky paint revealed by sanding to dry out.
Wet sanding removes unwanted impasto, removes surface gloss and creates a uniform satin texture, and produces a surface that is easy for the next layer of paint to adhere to. It's often a good idea for indirect painting. Wet sanding creates an inviting surface that really feels good to paint on.