RAR archive format supports archive locking. The locked archive cannot be modified by WinRAR. You may lock important archives to prevent their accidental modification.
Use “Lock archive” command in WinRAR shell mode or command “K” in the command line to lock an archive.
RAR_Unlocker.rar
Is this a RAR password unlocker? Or something else?
Something else.
From the WinRAR help: