The undo function is used to reverse a mistake, such as deleting the wrong word in a sentence. The redo function restores any actions that were previously undone using an undo. Some people may refer to this feature as a reverse undo.
For example, if you typed a word, and then deleted it using an undo, the redo function restores the word you deleted ("undid").
If the redo option is available, but nothing was undone, the redo option does nothing.