Complex sentence: What makes this cadence a "tierce de Picardie" is shown by the red natural sign--instead of the expected B flat (which would make the chord minor) the accidental gives us a B natural, making the chord major.
Split into simpler sentences: What makes this cadence a "tierce de Picardie" is shown by the red natural sign. Instead of the expected B-flat (which would make the chord minor) the accidental gives us a B natural, making the chord major.