Carbohydrates that can reduce Tollens' reagent or Fehling's solution are called reducing sugars. All monosaccharides (like glucose) are reducing sugars. In disaccharides, if the reducing groups (aldehydic or ketonic groups at the anomeric carbons) are free, the sugar is reducing (e.g., maltose and lactose). However, in sucrose, the two monosaccharide units ($\alpha$-D-glucose and $\beta$-D-fructose) are linked through their respective anomeric carbons (C1 of glucose and C2 of fructose). Since both reducing groups are involved in the glycosidic bond formation and are not free, sucrose is a non-reducing sugar.