When learning arpeggio patterns, the best way to start is getting familiar with right hand finger names and learning them. "p , i, m, a" letters are abbreviations of right hand finger names* which have come from Spanish words:

Hand	Letter	Name	Finger
	p	pulgar	thumb
	i	indice	index
	m	media	middle
	a	anular	ring

Arpeggio is an another playing technique of the guitar besides using strums. When playing chords on guitar, arpeggio formulas are a way to broke chords\' notes. Since the guitarists play notes one by one, it gives a harmonic effect to chords in different and exciting ways. Arpeggio playing also gives a chance to create dozens of possibilities on rhythmic arpeggio patterns. * These right hand finger names are for the guitarists who play right handed. For left handed guitarists, these are accurate for left hand. *