The 'cap' in winemaking - is the layer of grape skins, pips and other solid matter that rises to the surface of the wine during vinification.
When making several different styles of red wine, grapes are put through a crusher and then transferred into open fermentation tanks (made of wood or stainless steel). Once fermentation begins, the grape skins are lifted, raised to the surface by the carbon dioxide C02 gases which are released in the fermentation process.
So this layer of grape skins and other suspended solids is known as the cap. As the skins are the source of colour, flavour, aromas and the tannins - the cap needs to be macerated (mixed in) through the liquid several times each day, or punched / plunged down.

 

This is traditionally done by plunging through the cat, known as 'Pigeage' - a French winemaking term for the traditional plunging of grapes in open-top fermentation tanks. The cap of grape skins and pulp floating on top of the juice in red-wine fermentation inhibits flavour and colour extraction, may rise to an undesirably high temperature, and may acetify (turn to vinegar) if allowed to become dry. Such problems are avoided by submerging the floating cap several times a day during fermentation, to drown aerobic bacteria and encourage cuvaison (contact). This operation is comparatively easy with small fermenters, becomes much more difficult with large fermentation tanks.
A technique to extract these key red wine components is done by periodically pumping juice from the bottom of the tank over the cap. However, fine wine producers, to enhance the extraction of colour, flavours and tannin from the skins, often prefer to punch/ plunge down the cap in order to submerge it into the wine, or even to drain the wine from the tank and then splash it back over the cap to encourage greater circulation of the cap throughout the wine (a technique known as 'rack and return').
Whatever method is chosen, fine winemakers often extend the maceration period beyond the end of fermentation to encourage the full extraction of colour, aroma, flavours and tannin. Once this process is complete, the wine is pressed off the skins and transfered to oak barrels or stainless steel tanks, to commence the next stage in the winemaking, aging process.