What is the meaning of shading in photographic works?

The treatment of light and shadow is often expressed as "shadow tone" in photographic works. Generally speaking, photos dominated by white and light gray close to white are "high-key", while photos dominated by black and dark gray close to black are "low-key", and the middle tone is in between. High-profile makes people feel happy, bright and pure, so it is often used by people who shoot children's themes, medical work and beautiful scenery; Low-key makes people feel solemn, serious and gloomy, so it is often used when shooting portraits of old people or grand scenes.