Reducing sugar is any sugar (all monosaccharides, some disaccharides, oligosaccharides, and polysaccharides) that is capable of acting as a reducing agent because it contains free aldehyde group or free ketone group.
Aldehyde group or alkanal is an organic compound containing formyl group. The formyl group is a functional group consisting of a carbonyl center bonded to hydrogen and an R group. This group can be readily reduced to primary alcohol with the help of catalyctic hydrogenation either applied directly or by transfer hydrogenation.
Ketone group unlike aldehyde group does not have a hydrogen atome bonded to the carbonyl group but it can still be hydrogenated.