"Undo" allows you to revert an image back one step in the drawing or editing process. Almost anything you do to an image can be undone in this way. Further undo operations may be performed according to the number of undo levels configured in the Environment page of the Preferences Dialog. See the section on Undoing for more information about GIMP's very sophisticated "undo" functions.

The operation that has been "undo" is not immediately lost: you can get it back by using the Redo right after. But if you do another operation the "undo" will be lost definitively.