Complex sentence: The luminosity function or luminous efficiency function describes the average visual sensitivity of the human perception of brightness ("which of two different colored lights is brighter?") to light of different wavelengths.
Split into simpler sentences: The luminosity function or luminous efficiency function describes the average spectral sensitivity of human visual perception of brightness. It is based on subjective judgements of which of a pair of different-colored lights is brighter, to describe relative sensitivity to light of different wavelengths.