Red wines as Cahors wine must be racked regularly during their ageing, that means to be transferred from one tank to another to eliminate the lees (dead yeast and bacteria from the fermentation, organic precipitates, ...) that settle to the bottom of the vats by gravity, a sort of natural filtration and gentle that do not damage the wine. Racking will also prevent unpleasant taste transmited by lees.
Note that for some white wines, ageing is carried out upon lees to bring fatness and flavors (Muscadet or Champagne).
Racking also oxygenate the wine and improves the wine by reducing the concentration of carbon dioxide from fermentation, stabilizing its color (the Oxygen will "bind" the anthocyanins to tannins, both present in very large quantities in Malbec wines), or most importantly, helping the formation of the bouquet of aromas and flavors of wine ageing.
On the photo, you can see a racking for Château Paillas vintage 2008, the wine flows from the tank in a vat where it is pumped to another tank. Note the high position of the valve that only allows the clear wine to flow, the lees settle to the bottom of the tank will be eliminated through the door below.