State of the image is saved before any edit operation in so-called "Undo" history. Length of the history, that is, amount of stored image states is only limited by disk free space. The history is maintained throughout "lifetime" of the window and is cleared when the Image Editor window is closed.
The Undo command discards last edit action and reverts image to its previous state. This command can be repeated until the beginning of the Undo history is reached.
The command applies last discarded edit action again. This command can be repeated until the end of the Undo history is reached.
See also: