It shows how much the signal is being normalised by.

The gauges are designed to help set the optimum output level from your aux device. Ideally the gain (one to the right) should remain somewhere in the middle.

They are available in AC power mode as some people (including myself) have stuff fed into the aux in via the docking connector when not in the car. Some future applications may make it very attractive to have the aux input available when not docked in the car.....D........J...........