The undo function is very useful in a situation where you have made a mistake and you want to correct it fastly.e.g. you've written your whole text and you have erase it by mistake.
You can activate the undo function in two ways:
After you activate the function it will go to the last thing you have made so for example: you've wrote a whole chapter in your text and you've highlighted it and than deleted it by mistake. You activate the "Undo" function and it goes to the moment when the chapter was highlighted but before the deleting. You may of course go many steps back, not just one step.
The redo function is the exact opposite of the undo function. If you used the undo function and than for any reason decided to go back to the state before the undo you must use the redo function. To activate it press on the toolbar.