What it is
The rate at which pitch rises and falls periodically around the center note, in Hz — how many back-and-forth cycles per second. It measures how fast the wobble is.
Where it comes from
An envelope analysis of the F0 contour: extract the sinusoidal rise and fall of F0 over time and count the complete cycles finished per second.
How to read it
Well-trained singers usually land at 5.5–7.5 Hz (5.5–7.5 round trips per second). Below 5 Hz it sounds like a wobble; above 8 Hz like a tremolo or a bleat. Rate is hard to change deliberately — it is mostly a by-product of a well-coordinated phonation system.