If you want to open up a .rar archive file, File Roller, which is the default archive manager, won't be able to open the rar archive. The usual types that can be opened are: tar, gzip, bzip, bzip2, lzma, zip, jar, iso (read-only). We need to install unrar-free to be able to use the same archive manager with some other archive types, like: rar and 7zip.
sudo apt-get install unrar
That's it! Now you can unrar files with File Roller.