Definition - What does Sour mean?
The sour note on a wine is usually too overwhelming to the taster. A sour wine can occur during the making of a wine if it might have come in contact with acetobacter aceti, sometimes called VA or volatile acidity. This can turn the wine into vinegar, making it unpalatable.
WineFrog explains Sour
A sour wine is a wine which is out of balance. The possibility of coming across a sour wine is rare, but if found, it is very recognizable. The taste and mouthfeel of a sour wine can be similar to biting into a lemon or even accidentally consuming unpasteurized apple cider past its expiration. Such cider would have already started turning into vinegar. This can occur in a wine exposed to an acetobacter, but it can also occur if a wine is improperly stored in high temperatures for a long period. It may even be past its age for consumption. This can occur especially in some white wines.