Colour space
The agreed rules that turn colour numbers into real colours on a screen, such as sRGB.
See it in Color intelligence. Load a picture and export its colours as CSS variables: those codes are sRGB colours.
In plain words
The numbers 255, 0, 0 mean full red, but which red? A colour space answers that. It fixes exactly which red, green and blue the numbers point to, and how bright each step is. Most of the web uses sRGB. Newer screens can show Display P3, which reaches more vivid colours.