The detector uses normalized autocorrelation to find a repeating period in each short microphone sample, then maps that frequency to the nearest note in 12-tone equal temperament. The pitch trace shows whether a held note is settling, drifting, or moving with vibrato.
This is a monophonic detector: it works best with one sustained pitch at a time. Chords, speech consonants, percussion, room noise, and strong overtones can make the displayed note jump because they do not present one stable fundamental.