Sure, vielleuse
From the workshop that I attended, again about 35 years ago, I then understood this to mean that the singing voice did not produce a pure (as in sine wave) tone but, like most instruments, produced on that had harmonics or overtones of the fundamental note in the sound.

From other things I've read since I gather the production of overtones in one's singing voice is quite normal and, among other things, gives one's voice its characteristic timbre. What was intriguing to me at the time of the original workshop was the possibility that the context of one's learning had such influence over the content of what was learned.

While it would come as no surprise (to those who know me) that this has been a fundamental (pun intended) of my approach to teaching/learning style and associated activities I was interested to learn of a demonstration of its effect so simply.

Cheers, Rowan