The D property is how much dampening will be applied to the torque used to face the RocketPropulsion/Target|Target. When the part approaches the goal orientation it needs to decelerate, otherwise it will rotate past the goal and have to stop and re-accelerate back toward the goal. This is often creates undesirable rubber-banding effect, so applying dampening using this property is how that effect is avoided. The higher this value is set, the greater the dampening curve becomes, or the slower the part will approach the goal orientation.