Arpeggio picking is where you play the notes of a guitar chord individually instead of all at once as you would do when strumming guitar. On electric guitar you can use a technique called sweep picking (aka economy picking) to perform arpeggio shapes at supersonic speeds but on acoustic guitar we normally arpeggio pick through chords in an even eighth note rhythm. This technique is very useful as an accompaniment to ballads and slow tempo songs such as "Wonderful Tonight" by Eric Clapton.
The basic premise is that you hold an open chord and pick each string down individually allowing the notes to ring out and sustain uninterrupted. Upon completing 4 notes downwards you then reverse the pattern and pick 4 notes in an upwards direction. Once you become fluent in this motion you'll see that it cycles around and around in a never-ending circle. I would regarding learning how to arpeggio pick as a fundamental skill on guitar and one that you'll find highly useful in all your song playing.
It's possible to arpeggiate chords with many different patters however here's the one you'll find most common. Let's take a C chord for example. Starting on the 5th string, where the root note of the chord is located, you would pick down in a straight line 4 notes... so that would be the 5th, 4th, 3rd and 2nd strings. Upon reaching the 1st string you would then switch your picking strokes to upstroke's and pick in the opposite direction across the chord - 1st, 2nd, 3rd and 4th strings. This would then complete the circle.
As with all new techniques on guitar you'll make the fastest progress by practicing the slowest - strange as that may sound! Give your mind plenty of room to record what you are trying to learn and play through your arpeggio picking exercises perfectly and without mistake. Then you'll be able to gradually speed up more and more without making errors.
One you'll have to be aware of is that arpeggio picking chords will really expose the quality of your chords. In other words, if you are holding your chord and one or more notes are not being fretted correctly then arpeggio picking will show this up immediately. It's important therefore to spend time making sure that every string you hold across all your chords is ringing out clear and perfectly. While strumming chords will mask errors in your chords to a certain extent, arpeggio picking will advertise it for all to hear!
As an arpeggio picking exercise you should run through all of your open chord shapes and check them string by string to make sure they are all sounding cleanly. Whenever you find a string within a chord shape that is not working examine it closer in an attempt to find out why it's not sounding correctly and do your best to fix it. This kind of attention to detail within your chords will pay dividends when you're arpeggio picking on guitar through chords in songs.
Once you have mastered the technique of arpeggio picking you can also do hybrid variations where part of your chords are strummed and the other parts are picked with individual strings. This hybird is another very useful tool to have in your arsenal for playing guitar songs. Not all acoustic guitar lessons will feature instruction for this kind of technique but you will find video guitar lessons on Guitar in a Nutshell showing you how to do it.
