Hard light is light that creates shadows with a sharp or negligible transition from light to dark. The light that creates it is strongly focussed from a small (or relatively small), single-point light source. Hard light is found where the lighting is direct, undiffused, and is not bouncing or scattered by local objects or conditions. The flash on your camera is a hard light source. When direct and undiffused by clouds the sun is also a hard light source. A hard light source is small (relatively) and/or distant.