Glucose is a reducing sugar because it belongs to the category of an aldose meaning its open-chain form contains an aldehyde group. Generally, an aldehyde is quite easily oxidized to carboxylic acids. So if we use a mild oxidizing agent and react with glucose it will reduce it. Besides, if we go through the definition of reducing sugar, it states that it is a type of sugar that reduces another compound and gets itself oxidized. So glucose definitely fits this condition.
Also Read: Glucose Structure
Glucose is also a monosaccharide and thus is reducing in nature. It can reduce Fehling and Benedict reagents to cuprous oxide. The aldehyde group is further oxidized to carboxylic group producing aldonic acid. Thus, the presence of a free carbonyl group (aldehyde group) makes glucose a reducing sugar.
If we take the examples of α-D-glucose and β-D-glucose, these contain a hemiacetal group. When they are reacted with water they showcase an open-chain form which again contains the aldehyde group.