The "Stack" refers to the set of Layers which forms an
    image. The layers are arranged one on top of another.  The
    stack functions allow you to alter the order in which the layers
    appear. You can either raise the layer by one place, lower it by
    one place, raise the layer to the top of the entire stack, or
    lower it to the bottom. Layers can also be moved by dragging the
    layer up or down in the layers dialog, or
    by using the  and
    and 
     icons.
    icons.