Click on a pixel with this tool selected to fill all contiguous pixels with the active color.
Enable this option is you want to fill all like-colored pixels in the composite image. If the option is disabled, only the like-colored pixels in the active layer are considered. Note that, even if this option is enabled, the color is painted on the active layer only.
The tolerance, which can be set to values from 0% to 100%, defines how similar in color a pixel must be compared with the pixel you click in order to be filled. A value of 0% will fill only those contiguous pixels of exactly the same RGB color. 100% will cause all pixels to be filled.