Maximum efficiency is achieved when, upon release, the arrow will fly with minimum oscillation, displacement, and vibration and consistently hit the spot at which it was aimed at the instant of release. Although some arrows may consistently hit the target, arrow flight may be poor. Properly tuned equipment will yield clean arrow flight as well as consistency in grouping. The later will provide better arrow speed, better trajectory, better penetration, and suffer reduced impact from environmental effects. Broadheads demand super clean arrow flight or grouping will suffer significantly as the arrow fights the Broadheads tendency to windplane.
Timing refers to the relationship of one eccentric to the other.
Single cam bows obviously don't have the timing problem. Bows will come from the factory roughly timed and often require additional work. Poor timing will usually display itself by feel. As you draw the bow near full draw, the stop should be felt as a single solid point. If you feel two distinct, small bumps as your bow reaches full draw, your eccentrics are out of synch and must be adjusted.