During digestion, all digestible carbohydrates other than fiber break down into sugars. The body ultimately converts most of them into simple sugars, especially glucose, which cells can absorb and use for energy.
Some carbohydrates are already simple sugars, such as glucose, fructose, and sucrose. Others are starches made from long chains of sugar units. Enzymes begin digesting starch in the mouth, continue the process in the small intestine, and split larger molecules into absorbable sugars. The small intestine then transports these sugars into the bloodstream.
Fiber is the important exception because human digestive enzymes cannot break down most dietary fiber. It passes largely intact into the large intestine, where some types can be fermented by gut bacteria. That is why nutrition discussions often distinguish total carbohydrate, sugars, and fiber.
“Sugar” here describes the chemical end products of digestion, not necessarily added table sugar. A potato or slice of bread contains starch rather than much free sugar, but its digestible starch can still be converted into glucose.