Tone mapping
Squeezing a wide range of brightness into what a screen can show without losing the highlights.
See it in WebGL playground. Under Post-processing, change Color grade and watch the brightest parts.
In plain words
A 3D scene or an HDR photo can hold light far brighter than a screen's white. Cutting everything above white turns bright areas into flat blobs. Tone mapping bends the brightness instead, so highlights are squeezed but keep their shape. Dark and middle tones change only a little. Cameras and games do this on every frame.