The fermentation process has been used for centuries to produce or preserve foods. The fermentation process often involves the use of yeast and some kind of carbohydrate, e.g. sugar, as an energy source.
In breadmaking, it is common to add a small quantity of sugar to the dough tokick-start the yeast as it begins creating carbon dioxide. In baking, sugar feeds the yeast; the yeast’s enzymes transform the sugar into alcohol and carbon dioxide. For this reason, sugar is not always present in the end product.
A little sugar (or syrup) makes the yeast ferment faster and more effectively than when no sugar is added. The carbon dioxide makes the dough raise and the bread porous.