This tool is useful for copying regions of pixels between layers, or within the same layer.
In order to use the clone stamp, you must first choose an area to copy from by clicking an area of the image while pressing the Ctrl key. You may choose a new source area at any time by doing this again.
You may also set the size of the clone stamp brush by adjusting the Brush Size in the toolbar.
In the picture below, the orange has been cloned to appear as part of the apple.
While you are drawing, a circle will be drawn to indicate where the pixels will be copied from.
After resizing the cloned pixels, we have an apple with a stripe of orange.