Chiaroscuro is Italian for light (chiaro) and dark (scuro), which is an important aspect of both value and space. It describes the illusion of depth that is created when implied light and shadow are incorporated in a two-dimensional work. Some also refer to this as modeling. When light hits an object it creates highlights and shadows. The gradual change from light to shadow is what makes is seem rounded. This effect is normally most effective in paintings that feature implied smooth surfaces (like skin) and implied textured surfaces (like clothing). In this work by Caravaggio the light and shadow on David's face as well as the gradual change from light to shadow on his left (our right) side shows the effects of chiaroscuro, as do the folds in his clothing.
This can also be experienced by looking in a mirror and noticing how the subtle light and shadow on your face and clothing suggest your three dimensional form in a two-dimensional mirror. (example) However, as this is a photograph it would not be considered an example of chiaroscuro, as chiaroscuro is a technique used to imply depth rather than capture it in an photo.