Demi Sec literally translates as half dry. It's a medium sweet Champagne style and is increasingly popular. The amount of sugar (dosage) added after the second fermentation dictates the sweetness level of champagne. Demi-sec champagne has between 33-50 grams of sugar added per litre (g/l).
The Demi-Sec style also has a historal connection in that 19th century consumer tastes as a whole, enjoyed very sweet wine. It's a style that pairs best with dessert.