Undo as name suggests undo renaming changes you did.
Undo feature in File Renamer works using log file. That means you can undo any renamed operation which log file you have. That's why File Renamer save all logs automatically.
Press Undo Button and you can see below dialog box.
Now select log file you want to undo.
After selecting valid log file please press 'Undo' button. File Renamer will undo all renamed files in that log file.