Tone mapping
Squeezing a light value with an enormous range down into the narrow band a display can show, without clipping highlights into flat white.
An HDR renderer computes light in physical units where the sun can be thousands of times brighter than a shadow. A display handles far less, so an operator compresses the range with a curve that keeps detail in both the bright and the dark. Reinhard et al. (2002) gave a simple one; then came filmic curves like Hable's for Uncharted 2 and the industry-standard ACES.
The step sits last in the pipeline, after all light has been summed but before the gamma correction that encodes for the display. Swap the curve and the whole mood of the image shifts — choosing an operator is as much artistic as technical. The eye's own adjustment is mimicked with "eye adaptation" that slides the exposure as you move from dark to daylight.