“Light is the chaser away of darkness. Shade is the obstruction of light” (Leonardo da Vinci).
Chiaroscuro is an Italian term which means “light (chiaro) and dark (scuro). It creates an illusion of three dimensionality by creating a contrast between the shadows and the lit areas.
Just by drawing or painting the shaded and lit area you can make your subject magically appear three-dimensional in the real space of the two-dimensional flat surface that is the paper/canvas.
Leonardo da Vinci used Chiaroscuro to give his figure the impression of three-dimensionality, whereas Caravaggio used it more for the purpose of theatrical drama.
“Glossary”. The National Gallery, https://www.nationalgallery.org.uk/paintings/glossary/chiaroscuro [accessed 11/02/19]