RAR archive format allows to lock archives. WinRAR cannot modify locked archives, so locking important archives prevents their accidental modification by WinRAR. Such protection might be especially useful in case of WinRAR commands processing archives in groups.
Archive locking is not intended or able to prevent modification by other tools or willful third party. It implements a safety measure only for accidental data change by WinRAR.
Use "Lock archive" command in WinRAR shell mode or command "K" in the command line to lock an archive.