A .tar file (called a tarball) is a file which contains files and/or folders. Like a .zip but uncompressed. We use .tar instead of .zip, because it's faster to create.
Once you've downloaded and extracted the tarball, open the index.html file, then you can click around like normal. :D
Install 7-zip, and extract the tarball.
Open your terminal, and navigate to the Downloads folder:
cd Downloads
Extract the tarball:
tar xvf chadnet-wiki.tar
If the file is not found, recheck where you downloaded the tarball and navigate to the appropriate folder.