By default WordPress will turn off all debugging messages so the visitor to the site can't see any error messages showing anything wrong with the code.
If you do any WordPress development such as new plugins or theme development then this can cause a problem as you won't see if you have any of the settings wrongs.
Turning on debug mode will display all warnings or error messages you have with any installed plugins or themes. This makes it easy to spot if any are out of date or using old WordPress functions which are not supported any more.
Turn On Debugging
To turn on debug messages you need to open up your wp-config.php file and find the line.
Comment out this line a replace it with.
define('WP_DEBUG', true); define('WP_DEBUG_LOG', true);
You must remember to turn this off when you put your website live or the visitor will be able to see all your debugging messages.