Simply put, it is adding sugar to the grape juice to increase the potential alcohol of the wine.
It is often used in northern Europe which sometimes stuggles to ripen its grapes in its cooler climes. Another technique, enrichment, can be used to increase the potential alcohol of a wine – this is the addition of rectified concentrated grape must (RCGM).
The addition of sugar or RCGM is falling, perhaps due to climate change.
