"Much like OS X and Windows on the desktop, iOS on the iPad has an “Undo” and “Redo” option. Undo does just what it sounds like, it undoes the last text based action. For example, if you typed out a sentence but decided it wasn’t what you wanted to say, you could hit “Undo” and it would instantly disappear. Redo is also fairly self explanatory, as it redoes the prior text action that was just undone. For example, if you wanted to restore that sentence you typed out but made disappear with “Undo”, then hitting “Redo” would make it appear again."
Via John Evans