Undo
The undo function is very useful in a situation where mistake was made and it needs to be canceled e.g.
You finished writing the text which has been deleted afterwards.
You can activate the undo function in two ways:
- by pressing the
button on the toolbar. - by pressing the combination of CTRL+Z on the keyboard.
After you activate 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 you must 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/you to go as many steps back as desired
Redo
If undo function was used and for any reason user/ you wish to go back to the state before the undo, the redo function should be used. In order to activate redo option, 
button on the toolbar needs to be pressed.