how does auto brightness work?
The Auto option enables the lightsensor of the device.
On some Samsung devices there remains a slider when the checkbox Auto is checked. With that slider you can set a delta (small change) in the brightness set by the data of the sensor. You can chose if you want it a bit brighter or less bright than the sensors standard value.

In short:
 - Box checked: Sensor defines brightness (+/- delta)
 - Silider in the middle: delta = 0
 - Slider to the left: delta = - amount of steps from the middle
 - Slider to the right: delta = + amount of steps from the middle