The "Undo" menu command returns the playlist to the state it was in before the most recently executed command was completed. The keyboard shortcuts set for this action by default are <Alt+BackSpace> and <Ctrl+Z>.
You can undo operations until you reach the last saved version of the playlist or the first operation with the recently created playlist.
Use the "Redo" command from the "Edit" main menu to re-execute the command that was undone and to return the playlist to the state it was in before the "Undo" command was executed. The keyboard shortcut set for this action by default is <Ctrl+Y>.