Servo dead band refers to the electrical width of the center point of travel.
For instance, a typical center point is 1500 microseconds.
With a deadband of 0, the servo would interpret a pulse of 1501 microseconds to be off center.
With a deadband of 1, the servo would consider 1499, 1500, and 1501 to be center.
With a deadband of 2, the servo considers 1498 - 1502 microseconds as center.
But why do we care? Well, if we have a single servo per control surface, we really don't (unless the deadband value is way out of whack). But if we have two or more servos ganged on a surface, it becomes important. Each servo may have a minutely different time reference. So one servo might see a pulse and call it 1500 microseconds, while a second servo sees the same pulse as 1498 microseconds.
If both of these servos have a deadband of 0 or 1, they will push against one another to get to their own preferred center. This, of course, is hard on the servos and drains the radio battery (or loads the BEC). So setting the deadband in both servos to 2 or 3 microseconds eliminates the conflict and cancels the electromechanical wrestling match.
Here is Hitec's much shorter version:
http://www.hitecrcd.com/support/faq...nd-width-and-why-do-i-want-to-program-it.html