CKEditor contains many useful functions for example undo and redo function.
"Undo" function is helpful when user needs to cancel previous changes.
You can activate the "undo" function in two ways:
After user activates the function it will go back to the last change that had been made e.g.: You have written a whole chapter in your text, highlighted it and then deleted it accidently. In order to restore it user may activate the "Undo" function which will take you back to the moment when the chapter was highlighted but has not been deleted yet. This option allows user to go as many steps back as desired.