What is image post-processing?

Good post-processing should be invisible, this is an important rule. When everything looks different from what the viewer could see in life, unhealthy criticism begins.
If the post-processing is correct, then it will help correct camera errors (optical distortion, white balance, etc.) and mistakes made during the photographing process.
This can happen according to several criteria:
poorly composed the shot,
overexposed or underexposed,
a lot of noise