Hard light creates shadows with a sharp edge. There is a negligible transition from light to dark. Hard light is created by strongly focussed light travelling from a small (or relatively small), single-point light source like the Sun, a focussed beam of light, or an undiffused light bulb).
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 relatively small and/or large and distant.