Adjusting Colour Curves
With the help of the different channel curves, the Colour Curves effect allows you to modify the brightness and contrast of an image. Depending on if you modify the input, the output values or a mix of both, you can darken the shadow of a picture, brighten the highlights. By adding a point to the middle of the curve you can also modify the medium values.
The Colour Curves node takes an image of any type as its input and outputs an image with different brightness and contrast parameters.