What is the Undo/Redo tool in the Project Explorer? How does it work?

The Undo/Redo tool keeps track of your map editing and enables you to reverse changes that you have made.  For example, if you move or resize any object in your map, if you add or edit symbols, lines, polygons or text to your map, if you delete any object or layer from your map - all these changes can be undone.  The only limiting factor in the Undo tool is that it only maintains the list of changes while in edit mode.  So if you move your map title block then display a new grid, you will not be able to reverse the title move using the Undo tool.