The full system path to public_html is:
/home/username/public_html or /home/username/www
"username" is your cPanel/Linux username
"www" is simply a Linux shortcut (AKA symlink) and some people find it faster to type... Save a few keystrokes!
In your FTP program, the top level folder that you'd see would be /username
You would then upload your index.html file to /username/public_html