Given an FFT chain, this measures the spectral centroid, which is the weighted mean frequency, or the "centre of mass" of the spectrum. (DC is ignored.)
This can be a useful indicator of the perceptual brightness of a signal.
an FFT chain.
A Blip oscillator is ideal for demonstrating this because the number of harmonics is directly manipulated: as the number of harmonics increases, the centroid is pushed higher. In the example, left-to-right changes the number of harmonics, but up-to-down changes the fundamental pitch; note the different effects of these two on the centroid.