This is an Italian term which literally means 'light-dark'. In paintings the description refers to clear tonal contrasts which are often used to suggest the volume and modelling of the subjects depicted.
Artists who are famed for the use of chiaroscuro include Leonardo da Vinci and Caravaggio. Leonardo employed it to give a vivid impression of the three-dimensionality of his figures, while Caravaggio used such contrasts for the sake of drama. Both artists were also aware of the emotional impact of these effects.