Playing a chord’s notes in sequence creates movement while keeping the harmony clear. Classical guitarists and pianists use arpeggios as accompaniment; synthwave and trance use fast synth arpeggios as a signature sound.
Arpeggiators in synthesisers automate this, cycling through held notes at a set rate and pattern. In chiptune, very fast arpeggios fake full chords on chips that could only play a few notes at once.
Using it in a prompt
Specify "16th-note synth arpeggio" for electronic energy or "flowing piano arpeggios" for a gentle, romantic accompaniment.