What is the CIE standard observer?

According to the trichromatic theory of colour vision an observer can match a colour stimulus with an additive mixture of three primaries. Therefore any colour stimulus can be specified by the amounts of the primaries that an observer would use in order to match the stimulus. The CIE standard observer resulted from experiments where observers were asked to match monochromatic wavelengths of light with mixtures of three primaries. The standard observer is in fact a table showing how much of each primary would be used (by an average observer) to match each wavelength of light.

The graph below shows the colour matching functions for the CIE XYZ primaries. These are literally the relative amounts of the three primaries that an average observer would use to match a unit of light at each wavelength.