If you have made drawing or editing changes to the image which you don't want to keep, the Undo command allows you to undo the last change and return the image to its previous state. Almost anything you do to an image can be undone in this way (with the exception of scripts, which deactivate this function). Further Undo operations may be performed, depending upon the number of Undo levels configured in the Environment page of the Preferences dialogue. See the section on Undoing for more information about GIMP's very sophisticated “Undo” functions.

The operation that has been “undone” is not lost immediately: you can get it back by using the Redo command right away. But if you perform another operation, the ability to “Redo” will be irretrievably lost.