How I can use rar-files ??? I know. I am really, really stupid.
If you're using a Windows computer, the easiest way would be to download WinRar, which can handle the files.
You can find WinRar here: http://www.rarlab.com/ (http://www.rarlab.com/)
Basically, it's an alternative (some say better) method to ZIP files, i.e. a way to compress files so they take up less space.