What does it mean to dress up and dress down?

To dress up means to wear fine, very smart or formal clothing. To dress down means to sharply criticize, chastise, scold someone. "The teacher really gave Mary a good dressing down for using her cellphone in class." Additionally, to "dress up" something might mean to verbally make something look better than it is, perhaps a nasty situation or action. "Let's not dress up Mary's actions as harmless nonsense; it disturbs the others in the class.