The History window is where all of the actions you have performed on an image since it was opened will be shown. It consists of a list of actions, and a toolbar for rolling back or forward through the effects of the actions.
Every time you perform an action that changes the image, or when you create or modify a selection, it will be reflected in the History Window. If you have undone any actions, they will show up at the bottom with a grey background and may be redone by using the Redo facility. If you perform a new action, any actions that are redo-able will be erased to make way for the new action.
This will undo every action that you have performed. If there are many actions in the list, this may take some time to finish.
This will undo only the last action you performed.
This will redo the last action that was undone.
This will redo all the actions that have been undone.