Standard video compresses brightness into a narrow range, so bright skies and dark shadows both get flattened to "close enough." HDR (High Dynamic Range) captures and stores a much wider range of brightness values, so a screen capable of displaying it can show genuinely dark shadows and genuinely bright highlights in the same frame without losing detail in either. It needs a compatible screen, source file, and often a compatible app — which is why HDR content can look worse than normal on the wrong setup.