During development of plugins or themes, as well as during deployment of a WordPress site, developers or system administrators may enable debug logs to log any PHP errors that occur.
WordPress makes use of the WP_DEBUG constant which is defined in wp-config.php. The constant is used to trigger the debug mode throughout WordPress. The constant is set to be false by default.
Developers and administrators may also enable the WP_DEBUG_LOG and WP_DEBUG_DISPLAY companion constants to WP_DEBUG. WP_DEBUG_LOG creates a log file in the wp-contents folder, while WP_DEBUG_DISPLAY controls whether debug messages are shown inside the HTML of pages or not.
Any of the above will be useful while a theme, plugin or site is in development, however, if enabled on a production website, it might cause information disclosure – allowing malicious users to view errors and additional logging information. The WP_DEBUG constant should be disabled on production systems by either removing the constant from the wp-config.php file, or setting it to false as follows.
define( 'WP_DEBUG', false );
Read the entire article on WordPress Security