Adjusting the levels is an easy and powerful way of correcting for lighting levels and for giving an image a lift.
Image Optimizer computes and displays a histogram of the image so you can see the distribution of intensities in the image.
You can press the Auto Levels button: 
to have Image Optimizer set the levels automatically or you can drag the diamonds below the histogram until you have achieved the desired effect. The three diamonds correspond to the levels for: Black – shadow, Grey – midtone and Clear – highlight.
For example the image on the left is transformed into the image on the right by moving the diamonds as shown:
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Equalization
Whereas levels is used to stretch the histogram to make better use of the full range of intensities available equalization can be used to change the distribution of colors by evening out the distribution of intensities. To equalize an image simply drag the Eq. slider to the right.
Advanced usage
You can adjust the levels on RGB channels individually by pressing the red, green or blue buttons and then adjusting the position of the diamonds. The Auto Levels button will also work in this mode by automatically adjusting the levels or red, green and blue separately.
Working on RGB channels separately can introduce a color shift that you can remove with the Color correction tool.
See also: