Racking is an essential part to making any sound wine. It is a process that, on average, should be performed 2 to 4 times throughout the winemaking process. Doing so in a timely manner will aid in the clarification of the wine and help to inhibit the production of unwanted off-flavors.
But put very simply, racking means to siphon the wine must from one container to the next, so as to leave any sediment behind. In fact that is the sole purpose of racking, “to leave the sediment behind.”
Why are we concerned about getting the sediment away from the must in such an timely manner?
With the exception of some solids that may have settled from the fruit, most of the sediment is the result of dead or inactive yeast cells falling out of the must.
Towards the end of fermentation there is a fairly sizeable group of healthy, active yeast cells floating throughout the must that are running out of food (sugar). Once all of the sugars have been consumed, this active, now-starving group of yeast will start a process that can only be described as cannibalization.
This process can produce off-flavors in a wine that range from bitter, to rubber, to even metallic.
Adapted from www.eckraus.com/wine-making-racking/