The Undo command, also known as Command+Z, allows you to take back the last action in many cases. You can use it to erase mistakes while editing a document. But you can also use it in many other apps and situations. The Finder allows you to use Undo on deletions, moves and renames. You can often use Undo multiple times to take back multiple steps. You can also use Redo to step forward after an Undo. However, you cannot usually Undo after you have closed a document and reopened it.