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Understanding pitch accuracy
Piano identifies the fundamental frequency of each note you play from the raw audio the microphone captures, then maps it to the nearest note name and compares it against what the reference performance expects at that point in the piece.
A note within a few cents of the target pitch counts as correct; further off registers as partial accuracy rather than a flat pass or fail, since real playing — especially on an acoustic instrument — is never perfectly in tune to the cent.
Chords are harder than single notes, since several fundamentals overlap in the same audio window. Common chord voicings are handled well; very dense chords with many closely spaced notes are the current edge of what listening-based detection reliably separates.