What is the metering of SLR cameras?

Photometry You can imagine the pupil of human eyes and automatically adjust the amount of light entering the eyes, so that we can see the world clearly.

When the external light is too strong, the pupil shrinks and the amount of light entering the eyes decreases, so that the objects we see are normal, not white. When the external light is too weak, the pupil increases and the amount of light entering the eyes increases, so that the objects we see are not a mass of black.

This is basically the case with photometry. The camera adjusts the amount of light entering the lens by metering, so that the photo looks like the same scenery, not a piece of white or black.