What it is
How many times per second the vocal folds complete a full vibration, in hertz (Hz). Singing A4 means the folds open and close 440 times per second — F0 is 440 Hz. The ear perceives it as “pitch”: double the F0 and the sound rises by an octave.
Physically, any periodic signal can be decomposed into a harmonic series (the fundamental plus overtones). F0 is the first and lowest of these, and the spacing between adjacent harmonics also equals F0.
Where it comes from
Read it from the spectrum or spectrogram: take an FFT of the current frame, and the first line of the strongest low-frequency harmonic series is F0; the spacing between harmonics is corroborating evidence.
It can also be measured directly from the waveform period: F0 = 1/T. SonaLab shows it in three places at once — the spectrum, the pitch chart and the spectrogram.
How to read it
Male speech sits around 85–155 Hz, female speech around 165–255 Hz; a bel-canto tenor’s High C is about 523 Hz (C5).
A steady F0 does not guarantee a beautiful sound, but a jumping F0 almost always means the phonation control is off. In Mandarin, the tones are written directly into the shape of the F0 contour.