The tolerance defines how similar in color a pixel must
be (to the pixel you click) to be filled. Values can range from
0 to 255. A low tolerance fills pixels within a range of color values
very similar to the pixel you click. A high tolerance fills pixels within
a broader range.
To smooth the edges of the filled selection, select Anti-aliased.
To fill only pixels contiguous to the one you click,
select Contiguous; leave Contiguous unselected to fill all similar
pixels in the image.
To fill pixels based on the merged color data from all
visible layers, select All Layers.
Click the part of the image you want to fill. All specified
pixels within the specified tolerance are filled with the foreground
color or pattern.
If you’re working on a layer and don’t want to fill transparent
areas, make sure that the layer’s transparency is locked in the
Layers palette. (See Lock layers.)