Arpeggio means "like a harp 'and when a harp is playing chords, it is often a fast broken chord from the lowest to the highest. We can make a similar effect on the piano. Listen to the intro to 'Deep River' where two chords played as arpeggio.

Deep River (intro) Arpeggio mark


You can read more about arpeggio in 'Chords On Piano 1' page 56.