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.
While Chiarascuro - the 'light-dark' technique used to highlight though tonal contrast - is used in many different styles of art, it is Italian Renaissance and, particularly, Baroque paintings for which the technique is most well known.
The technique is used in both Caravaggio's and Artemisia Gentileschi's versions of the biblical story of Judith beheading Holofernes, the former said to have greatly influenced the latter's two slightly different compositions.
In Caravaggio's work, the technique appears to be used to highlight Judith's purity - her pure white dress in the 'spotlight' her elderly maidservant falling into the shadows. The light cast highlights the unnatural angle of Holofernes's head, showing us the ease with which Judith's sword is slicing though his flesh, as though she is aided by god. His body appears to be turning dark, the light emphasising his twisting body and pained expression on as a way to serve the viewer's understanding of his position as the evil force. The gore is kept to a relative minimum for a beheading scene.
No such purity is highlighted in either visual representation of the biblical myth in Gentileschi's versions. Rather, the highlight falls on Judith's steely determination. The force with which she plunges the sword down and with which her maidservant holds the Assyrian king's body, unnaturally contorted, against the bed. These are two women who are working to ensure the gory, bloody death of this tyrant. In the second version, a brief highlight on Holofernes's red velvet blanket serves to remind the audience of the audacious luxury of the Assyrian court and the formidable strength of Holoferens's army.
In Caravaggio's painting, the light comes from beyond the left hand side of the composition, but Gentileschi's versions both show the light as though is emanates from Holofernes himself - or more accurately perhaps from the hilt of Judith's sword. This bizarrely unnatural light gives the impression of the assent of god, demonstrated by Judith's radiant purity in Caravaggio's version.
There are, of course, issues about the circumstance of Gentileschi's painting - predominantly being forced by her father to marry her rapist.
Both painters - as well as many others - successfully employ this technique as a way to highlight (though both what is brightened and what is left dark) and provoke question (where is the light coming from? why are certain elements made light or dark? what is the artists trying to convey through each highlight).