The Brightness-Contrast tool adjusts the brightness and contrast levels for the active layer or selection. Levels and Curve tools are more efficient.
This slider sets a negative (to darken) or positive (to brighten) value for the brightness, decreasing or increasing bright tones.
This slider sets a negative (to decrease) or positive (to increase) value for the contrast.
The Preview checkbox makes all changes to the brightness and contrast dynamically so that the new level settings can be viewed immediately.