"Undo History" activates the Undo History dialog, which allows you to jump back and forth in the undo history of the image. This is especially useful when you work on difficult tasks, where you often need to undo several steps at once.

Click on a thumbnail to access to the corresponding step. "Undo" and "Redo" buttons bring no more. As for the "Clear undo History", it may be useful in case of a complex work to relieve your computer memory.