Nothing too complicated here. Move the Brightness slider to the right to increase the Brightness level. Move the slider to the left to make the image darker. The center 0 value produces no change to the original image. The Contrast slider works the same way. To the right = more Contrast, to the left = less Contrast.
One note here: A lot of graphics programs exaggerate the color when the Contrast is increased. The CheapSkate Contrast filter works on the black level of an image, which produces more pleasing results.
PSP Contrast 50%.
CheapSkate Contrast 50%.
The 2 adjusted photos above, had their Contrast exaggerated to a point beyond what is good for them, just to illustrate a point. The center photo above had it's Contrast raised to 50% maximum, using Paint Shop Pro. Even at 50%, it's apparent that the color is starting to get blown out. The problem becomes even more noticeable at higher Contrast settings.
The photo on the right had it's Contrast adjusted using the CheapSkate Brightness / Contrast filter. As you can see, their is no increase in the color saturation. This gives the user more control over the image, by leaving the Saturation level to the Saturation filter.