What is Col Fondo, why is it cloudy?  Most Prosecco is made using use the Charmat method, where the wine is fermented dry, then put into pressurized steel tanks called autoclaves, where it undergoes a second fermentation kick-started by a carefully measured dose of sugar and yeast.  The CO2 from this second fermentation is trapped as the bubbles we love.  The spent yeast cells fall to the bottom of the tank, and the wine is run off the top and bottled clear.

"Col Fondo" wines are made using the Methode Ancestrale, where wine is bottled before fermentation has finished, so any further CO2 created from it is trapped.  When the fermentation is complete, the producer will generally choose to simply leave the spent yeast cells in the bottle, causing the familiar cloudiness.