command fills the
image's selection with the solid color shown in the foreground part
of the Color Area of the Toolbox.
(The color is also shown to the left of the menu entry.) If
some areas of the image are only partially selected (for
example, as a result of feathering the selection), they are
filled in proportion to how much they are selected.
Please note that if the image has no selection, the whole active layer
Activate the Command
You can also fill a selection by click-and-dragging from the
Toolbox foreground color.