The racking of the musts (or unfermented grape juice) is a process that aims at clarifying the juice by gravity. The must is left in a tank, usually at cool temperature until the solid mater (skins, pips, pectic particles…) falls and sediments at the bottom of the tank. The clear juice is then pumped from the top. The heavy deposit can either be filtered or sent to the distillery. 

It is a crucial operation to obtain finesse in white and rosé wines. More details here. 

The racking of the wines (fermented) consists in removing the clear wine from its lees (the other name for the yeasts after alcoholic fermentation is completed). It is a natural and gentle way to clarify red and white wines. 

Traditional racking in Bordeaux