Levels is a tool that can move or stretch the intensity levels of an image histogram by redefining the "input levels" of a histogram so they are mapped to the "output levels" of the histogram. You can adjust brightness, contrast, and tonal range by specifying the location of complete black, complete white, and mid tones in a histogram.

1. Input histogram: This displays a histogram of the image. You can redefine the "input levels" using two sliders. The default values of minimum and maximum are 0 and 255 respectively.
2. Output slider: Define the "output levels" that are used to map redefined "input levels".

The immediate effect of the Levels tool is displayed in the image viewer When you like the effect shown in the image viwer, click Apply, then Levels tool will be applied to the entire image.