Explain Undo And Redo Commands (PC T1 Q21)

Undo command :- 
This command will return your document to its previous shape, before your last change.  If you choose to undo change number 4 it will also undo the first three as well

Redo command :-
The redo command will replace the action that you recently changed with the undo command.

The shortcut Key of undo command is ctrl+z.
The shortcut Key of  redo command is ctrl+y