The source–filter model of the voice: the vocal folds make a buzz rich in harmonics (the source), and the vocal tract shapes it with resonant peaks called formants (the filter). The vowel you hear is set entirely by where those peaks sit — F1 and F2 do most of the work. Here a buzz feeds a bank of parallel resonant band-pass filters, one per formant; pick two vowels and morph between them and you sweep a sung "aah → eee". Add vibrato and a breath of noise and it starts to sound alive.
sing a note (a s d f …) and sweep the morph — watch the formant peaks slide in the spectrum