The / var / www is the default hosting the content to use Apache to send the browser when requested. This directory has blocked the permissions so that only the admin user, ie root, you can write to that folder. This is for security measures and should not be changed on a machine that performs the functions of production web server. May help to change the permissions if we are to devote to web programming, because it simplifies access to that folder. In that case the following commands in a terminal we can make the change. Open a terminal by pressing CRL + ALT + T or from the menu Applications -> Accessories -> Terminal. The first command you must adapt to our case. Where it says user user change we want to have access to the directory:

sudo chown usuario.www-data /var/www/
sudo chmod 775 /var/www

With this we could write directly in / var / www.