PDF (adobe.com)

Levels overview

You use the Levels adjustment to correct the tonal range and color balance of an image by adjusting intensity levels of image shadows, midtones, and highlights. The Levels histogram is a visual guide for adjusting the image key tones. For more information on how to read a histogram, see About histograms.

You can save Levels settings as a preset, then apply them to other images. See Save adjustment settings and Reapply adjustment settings.

Levels dialog box

A.
Shadows

B.
Midtones

C.
Highlights

D.
Apply Auto Color Correction