Definition - What does Reduction mean?
Reduction is most likely to occur in a fermenting wine when sufficient oxygen is not present. When there is not enough oxygen, the wine becomes "reduced." An indicator which can help identify this is when the fermentation begins to smell like sulfur or rotten eggs. If noticed in time, this can be easily fixed by a pump-over or by punching down the cap.
WineFrog explains Reduction
For a successful and effective fermentation of grape juice to occur, it is important to have the right amount of oxygen present. (The exception would be if carbonic maceration is the desired fermentation technique.) Yeast is a living organism, and like any other living organism, it needs oxygen. Without it, a stuck fermentation might result or a wine can turn rotten. A stuck fermentation can be reversed, but a rotten wine cannot be fixed.
During fermentation, pump-overs and punch-downs of the cap are performed to properly aerate the wine. In calculated dosages, sulfur is also added to keep a desired amount of oxygen so oxidation does not occur. It is an important balance to keep.
With proper attention during the fermentation process, reduction is easily prevented.