The "cap" refers to the floating raft of grape skins that rise to the surface of the liquid during fermentation. The formation of a cap is usually the first gross sign of fermentation. The cap consists of the solid portions of your must, buoyed by the carbon dioxide (CO2) produced by fermentation. As fermentation progresses and the solids break down, the cap will become less and less dense and easier to break up by punching down. The WineCoach will prompt you to break up and mix the cap by punching down.