Wikipedia
Co-fermentation is the practice in winemaking of fermenting two or more grape varieties at the same time when producing a wine. This differs from the more common practice of blending separate wine components into a cuvée after fermentation. While co-fermentation in principle could be practiced for any mixture of grape varieties, it is today more common for red wines produced from a mixture of red grape varieties and a smaller proportion of white grape varieties.
Co-fermentation is an old practice going back to the now uncommon practice of having field blends (mixed plantations of varieties) in vineyards, and the previous practice in some regions (such as Rioja and Tuscany) of using a small proportion of white grapes to "soften" some red wines which tended to have harsh tannins when produced with the winemaking methods of the time. It is believed that the practice may also have been adopted because it was found empirically to give deeper and better colour to wines, which is due to improved co-pigmentation resulting from some components in white grapes.