The wp-config.php file contains configuration settings for a WordPress install. It is either created during installation or by copying the wp-config-sample.php file. The file contains database settings like the table prefix and security keys. Additional security can be added by moving wp-config.php out of the public directory and limiting its access. The file can also override database values and URLs. Developers can use SVN to version control WordPress sites and update core files, themes, and plugins automatically for testing. Customizing wp-config.php requires testing to avoid breaking plugins.