Reducing Sugars are those sugars which have a free anomeric carbon(the carbon of Carbonyl group present in Carbohydrates).Due to which the carbon of Carbonyl attains a partial positive charge and hence able to gain a electron from a electron donating specie.
In this way the reducing sugars have a reductive properties i.e. they can get oxidized and reduce the oxidizing compounds.
this is why that in Barfoed`s reagent and benedict`s reagent Cupric ion Cu+2 get reduced to cuprous ion Cu+.