The Eyedropper tool has only one function, but it is a useful one. The eyedropper can be used to "pick up" colors that you’ve already used in an image. Say you’re working with an image with many shades of Green. You want to pick a specific shade, but there are several that are so similar that it’s difficult for your eye to distinguish between them. Just click on the eyedropper, and click on the exact portion of the image that contains the color that you want. The active color for your paint tools will automatically change to the color that you picked with the eyedropper. Additionally, the Eyedropper will automatically switch over to the previously selected tool, enabling you to rapidly resuming work on the image with the color you just selected.

Tip:  You can use the Eyedropper tool to select color for the background color by using the right button on your mouse.