.rar are compressed files, you need to decompress them before you can use them. There are lots of different programs that can help with that for free, such as Winrar, Winzip, 7zip, etc.
You can find a free download of Winrar online pretty easy, like at https://www.rarlab.com/
It'll tell you that you "need" to pay to keep using it after 40 days, but for normal every day personal use, you can just ignore that (it just locks out some of the business oriented functions, nothing you'd need to use to just open .rar files).