Left: your sound split into its frequencies, scrolling from right to left. Brighter = louder. When you sing one note you'll see a stack of bright bands – the lowest is the note you're singing (harmonic 1), the ones above are its overtones (2, 3, 4…). Change the vowel (ee → oo → ah) and watch which overtones get stronger.
Right: each harmonic measured. The bar shows how strong it is, then its nearest piano note, and how many cents (hundredths of a semitone) it is above or below that note. Overtones don't land exactly on piano notes – e.g. harmonic 5 sits about 14 cents flat and harmonic 7 about 31 cents flat. Those are the microtones.
Overtone lines (in ⚙ Settings): traces each harmonic with its own coloured line (1 is white) so you can see exactly where each overtone starts and how it moves. The numbers on the right and in the panel use the same colours.
Spacing (in ⚙ Settings): “like the piano” makes each octave the same height; “by frequency (Hz)” gives every 100 Hz the same height (1k = 1,000 Hz), so the overtones look like an evenly spaced ladder.