Normally, grapes go into a destemming machine before fermenting. In the case of whole cluster fermentation, however, wine is made with whole bunches of grapes, including their stems.
Why ferment whole clusters? Whole cluster fermentation makes wine more complex by imparting spicy and herbal flavors and creating candied and airy fruit notes. The stems also add tannin structure and help smooth out high acidity.
Pinot Noir is one of the most common varieties to use whole cluster fermentation.