Now you look at the layout and make adjustments. More contrast. Less color. You check the negative space and select your lighting. You're revising now, shifting elements around until they meet your approval. Some artists do this in subsequent concept sketches, others paint on top of previous layers. The method doesn't matter; only the process.
Finally you go back and add detail. The delicate shading along the lower edge of a snowflake. The highlights in the dragon's iris. All those intricate touches that breathe life into your creation and make the audience quaver in astonishment -- That's polishing. It's just as important as the first two steps. The problem is that it cannot be done properly until after the larger elements have been arranged.
As a beginning writer I often tried to polish too early. It cost me a lot of time.