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 icons.