Balsamic reduction is just balsamic vinegar simmered until it becomes thick. Some people add sugar, honey, or some other sweetener but you do not have to. Simply pour the balsamic vinegar into a saucepan. Turn the heat to medium and bring the vinegar to a simmer, stirring frequently to check the consistency. Keep simmering until the glaze is thick like syrup and covers the back of a spoon. Remember, it will become thicker as it cools.