Threshold. The Threshold option controls the sensitivity of the color matching. If the Threshold is set low, the program will only fill colors that exactly match the color where you clicked. If the threshold is set high, the program will fill most or all pixels in the image.
This can be useful where you are trying to remove a muddy spot from the image. Using the Undo Button, you can try different levels of threshold until most of the muddy spot is filled with white.
The image to the right illustrates the process. The top image shows a muddy spot on the sketch map. The bottom image shows the muddy spot filled with white. Most of the spot has been removed, although the grid lines have also been removed. Setting a lower threshold would have removed less of the grid lines. Setting the threshold higher would get rid of the final traces of brown. The usual approach is experiment to find the best threshold and then finish cleaning with the Pen Tool.