config.php:The Config class provides a means to retrieve configuration preferences. These preferences can come from the default config file (application/config/config.php) or from your own custom config files.
autoload.php: If we find that we need a particular config file globally, we can have it loaded automatically by the system.
database.php: CodeIgniter has a config file that lets store your database connection values (username, password, database name, etc.). The config file is located at: application/config/database.php
routes.php Routing rules are defined in application/config/routes.php file. That permits to specify own routing criteria.