In Preview, what properties are the top three level sliders?
The control is a histogram, also called &quot;levels&quot;. When you adjust the sliders, you are adjusting the exposure of the image.
Photo editing technique is probably out of scope of Ask Different, but you can read more about it in Apple's documentation for Photos: https://support.apple.com/guide/photos/apply-levels-adjustments-pht362f9034f/6.0/mac/11.5