The cap (chapeau in French) is the layer of grape solids that forms on the surface during red wine fermentation.
I got to sample a grape and it was like eating pop rocks, because of the CO2 forming as a by-product of fermentation. The cap limits the oxygen available to the yeast, encouraging them to eat away at the grape sugar to form alcohol.
The cap must be broken up and mixed back in with the liquid below, however, in order to extract the phenolics, which add color, flavor and longevity to the wine. Phenolics can be found throughout the grape, but are particularly rich in the skins, seeds and stems (the solids!). On a smaller production scale, the winemaker will “punch down” the cap several times per day.
On a larger production scale, the winery will “pump over” the cap.
We recently had an illustration created to use in our classes to explain how pumping over works. Thanks, Laurel.