Answer:
When an acoustic guitar resonates it is basically ringing out all of the open notes on the guitar. Pluck a string on your guitar without muting any of the other strings, and you will get this effect. What happens is when the string vibrates and the sound gets sent through the guitar body, the string is also vibrating the guitar neck and therefore the other strings. It is a very subtle effect yet very noticeable and very effective. You can get this same effect with a piano if you press the far right pedal and hit a chord firmly, let it ring out and listen to the rest of the piano sing.