Chaptalization

The addition of sugar to wine must before or during fermentation in order to raise the potential alcohol content. (In the process of fermentation, yeast converts sugar to alcohol.)

It is legal and widely used in cooler European regions, where cold temperatures in a particular year can lead to grapes that don’t ripen fully, and therefore, produce thin wines. By increasing the wine’s alcohol content, the winemaker can produce a fuller bodied wine.

Chaptalization is not permitted in many warm wine regions, including California.
(The Wine Bible, Karen MacNeil)