You changed the database configuration for Cacti to the username and password that you created earlier. These settings will tell Cacti where to find the database and what credentials it needs to use to connect to it. You also changed the default URL path to fit your installation. As you install Cacti to /var/www/html/cacti, a sub-directory of the document root, you need to change this setting to /cacti/, otherwise Cacti will not work correctly.

For the poller to work correctly, Cacti needs a system user account. You are going to create one now and also set up the poller's cron entry.