The bows efficiency is actually is the tranfer of energy from the bow to the arrow. A more efficient bow will transfer more of it's stored energy to the arrow (assumingthe same weight arrow) and will not loose energyin other forms such as vibration or noise. A heavier arrow will absorb more of the bows efficiency.

Tuning is getting the most out of an arrow flight as possible froma bow. It usually involves adjustments to the bow and the arrow to insure the best possbile flight. A straight flying arrow is optimal with fletchings doing as little as possible to stabilize the arrow. A well tuned bow/arrow combo will also help in forgivingness in flaws of the shooter.

If you are asking how to tune your bow, I would suggest starting by downloading eastons tuning guide.