The Levels adjustment layer allows you to adjust the shadows, midtones, and highlights of your image. Add a Levels layer by clicking the Levels icon on the Color tab of the toolbar.
Levels Properties
Channel
The channel pulldown lets you pick a color channel to adjust.

Sliders
The black slider adjusts the shadows of your image.
Sliding the black slider to the right will make the shadows of your image more visible.
The gray slider adjusts the midtones of your image.
Sliding the gray slider to the left will make your image darker.
Sliding the gray slider to the right will make your image lighter.
The white slider adjusts the highlights of your image.
Sliding the white slider to the left will make highlights of your image more visible.