Definition - What does Cap mean?
The cap is the layer of stems, seeds and skins which float above grape must (juice) the following day after crush. The cap plays an important role during fermentation, as it contains the elements which impart color, aroma and flavor into the wine which is being made. This is done by pump-overs and punch-downs.
WineFrog explains Cap
Once grapes are harvested, they are sorted, selected, crushed and then sent to their designated fermentation vessels. The crushing of the grapes opens the skins, allowing the juices to run freely. After about 24 hours, the skins, seeds and other solids begin to float to the top of the grape juice and forms a "cap".
During the fermentation process, it is important for the cap to be integrated into the grape juice. This allows for an even fermentation and also prevents any wine taints and growth of unwanted bacteria. Wineries do this by pumping over the wine from a bottom valve into the top of the tank where the juice is then sprayed over the cap. Or it is done by using a tool which is manually used to push the cap into the wine. The latter process is used for small batches of wine.