What does it mean to reduce ash and add ash in the later stage of photography?

The reduction and addition of ash in the later stage of photography are

Adding gray patches can reduce the brightness of the camera as a whole, and can play a role in suppressing highlights. In outdoor high-light environment, gray items must be used. Ash reduction is the opposite.

Everyone knows the formula for adjusting exposure compensation: "white plus black minus".

This is related to the metering mode of the camera. There is a "18% gray principle" in camera metering, and the exposure will be controlled according to 18% gray.

Why 18%? A long time ago, scientists concluded that the average reflectivity of natural objects is 18%, so the camera can get basically correct exposure results based on this.