"Redo" is like the reverse of "Undo". It will reapply the last change that you made to the file you're working on. It doesn't seem to be a useable option most of the time, however. You'll find that it's grayed out most of the time, and not a particularly useful feature.

To redo the last operation:

    Choose Edit > Redo.

Hotkey is CTRL + Y.