The color curves layer allows you to control the tonal range of an image. Add a curves layer by clicking the Color Curves icon on the Color tab of the toolbar.
Curve Properties
Channel
The channel pulldown lets you pick a color channel to adjust.

Graph
The horizontal axis of the graph represents the input values of each color in your image, with dark values on the left and light values on the right.
The vertical axis of the graph represents the output of the effect, with dark values on the bottom and light values on the top.
The curve represents how the colors in your image are mapped from input values to output values.
Click anywhere on the graph to add a control point. The control point can be dragged to change the curve.
Steep sections of the curve will result in high contrast. Flat sections of the curve will result in low contrast.