The 
Redo button restores changes that you reversed with the Undo command. If you use undo to erase your most recent ink strokes and go too far, you can use redo to restore the strokes you want. Similarly, after recognizing an expression, you can use undo to look at your ink input again and then tap the Redo button to restore the formatted text.
You can also use the Redo gesture to invoke this command:
The Redo gesture is a stroke straight down and then straight up, away from other ink on the display.