Hey kiddo! Do you know what an arpeggio is? It's when you play notes in a row one at a time, like when you play the chords on a piano one note at a time.
A non-harmonic arpeggio is when you play notes in a row that don't fit together in a perfect way like a regular chord. It's like when you try to fit different shapes together, like a triangle and a square, instead of just squares together.
So instead of sounding pretty and happy like regular chords, non-harmonic arpeggios can sound a little weird or spooky. But they can also be used to make really cool music!