The Brightness - Contrast tool adjusts the brightness and contrast levels for the active layer or selection.
Brightness-Contrast Settings
The Brightness slider sets a positive or negative value for the brightness of the active selection or layer.
The Contrast slider sets a positive or negative value for the contrast level of the active layer or selection.
The Preview checkbox toggles the use of a dynamic update of the active layer or selection. If this option is turned on, any changes made to the Brightness or Contrast sliders will be seen immediately.