The undo function is for when a user needs to reverse a mistake such as deleting a shortcut icon or the wrong word in a sentence. The redo function restores any actions that have been previously undone using an undo.
For example, if you typed a word, and then deleted it using an undo, the redo function would restore the word that you deleted ("undid").
On the PC the shortcut key to redo is usually Ctrl+Y or Cmd+Y.
If the redo option is available, but nothing was undone, the redo option will do nothing.