In curves you can adjust any point along the 0-255 scale and keep up to 15 other values constant. Here the highlights are shown at the bottom left of the diagonal. This area describes the darker areas of your image. Knowing what numbers will print is important. Most images need considerable correction in the mid tone areas.
Dot gain is highest here.
The gray gradient can be changed by clicking on the arrows in the middle. If the gradient was reversed you would be editing in pixels values form 0-255 for each channel. I have provided a PDF file Working with curves for you to read or download.