Simply moving the mouse and clicking plays notes, chords and arpeggios.
Arpeggios are chords which are played one note at a time.
It is easier to play it than to describe it. Most non musicians have had either an unfortunate encounter with a bad music teacher, or never had the opportunity to make music with an instrument. No manual to struggle over, no complicated stuff at all. Simply play and learn. You will learn what it is like to PLAY music of your own.