If "66rar" refers to a specific archive file, you will need a utility to extract it.
Use WinRAR (the original developer) or 7-Zip (a free, open-source alternative). macOS: Use The Unarchiver or Keka .
You cannot open these natively in many operating systems without third-party software.
A RAR file is a compressed archive that holds one or more files in a smaller size than their original format.
Always scan compressed files with antivirus software before opening, especially if they come from unknown sources.