Color Gamut Mapping is the process a TV uses to match the colors in a movie or game to the range of colors the screen can actually display.
For example, a 4K HDR movie might have very rich colors, but if the TV’s panel cannot show them all, the TV maps them into its available range so they still look natural.
Better color gamut mapping means more accurate and lifelike colors, especially when watching HDR content.
See also: HDR, Wide Color Gamut, Color Accuracy