An accurate definition of tenebrism would be that it is marked by a very strong contrast between light and dark tones, imagine like when people tell ghost stories and everything's dark but they point a flashlight to their faces. The darkness is the dominating feature but the light is there as a sharp contrast. This is what tenebrism was and what Caravaggio did all the time.
The harp is an instrument to play, which is often used in concerts and orchestras for background music. They were first found as arrows in Egypt (3000 BC), but were developed into a musical instrument.
