The Weekly Walla Walla Wine Word for Dummies is: Chaptalization
Also known as "Brix Adjustment," this age-old winemaking practice is named after its
"inventor" Jean-Antoine-Claude Chaptal who suggested that by adding sugar to under-ripe grapes during fermentation, more alcohol could be produced.
This age-old technique predates the 19th century and even traced back to Roman times. Early vintners recognized the virtues of adding honey to their grape must. They may not have understood the science behind fermentation (yeast + sugar = ethyl alcohol), but definitely understood a good thing once they took a sip. Today, the most common type of sugar added is derived from cane.
However, this practice has some legalities and chaptalization varies on country, region and even some types of wine. Chaptalization is prohibited in Argentina, Australia, Austria, California, Italy, and South Africa. It is generally permitted in regions where grapes tend to have low sugar content, including regions in France, Germany (not including the production of German Prädikatswein aka high quality wines), and the United States.