A 'col fondo' style of Prosecco, or any Italian sparkling for that matter, is one where the lees (or yeasts) of the wine are deliberately bottled when the wine is put in bottle to prompt secondary fermentation, where the bubbles come from. This is quite different from the majority of Prosecco on the market, which are often filtered and made with the Charmat method, or creating sparkling in a big big tank. The col fondo tradition is older in Italy and represents the way all frizzante wines were made in the Veneto before more modern approaches. Decant the wine off the sediment into a carafe, if you like, or incorporate the lees by rolling the bottle if you prefer a cloudier style.