People who've used Elements for a while will tell you that the Levels command is one of the program's most essential tools. You can fix an amazing array of problems simply by adjusting the level of each color channel. (On your monitor, each color you see is composed of red, green, and blue. In Elements, you can make very precise adjustments to your images by adjusting these color channels separately.)
Just as its name suggests, Levels adjusts the level of each color within your image. There are several different adjustments you can make using Levels, from general brightening of your colors to fixing a color cast (more about color casts later in this chapter). Many digital photo enthusiasts treat almost every picture they take to a dose of Levels, because there's no better way to polish up the color in your photo.
The way Levels works is fairly complex. Start by thinking of the possible range of brightness in any photo on a scale from 0 (black) to 255 (white). Some photos may have pixels in them that fall at both those extremes, but most photos don't. And even the ones that do may not have the full range of brightness in each individual color channel. Most of the time, there's going to be some empty space at one or both ends of the scale.
When you use Levels, you tell Elements to consider the range of colors available in your photo as the total tonal range it has to work with. Elements redistributes your colors accordingly. Basically, you just get rid of the empty space ...