WP_DEBUG is a PHP constant (a permanent global variable) that can be used to trigger the "debug" mode throughout WordPress and display error messages.
To enable the debug mode, open the wp-config.php file at the root if your WordPress installation and set the WP_DEBUG value to true.
You should find this line at the end of the wp-config.php file above the line that says
/* That's all, stop editing! Happy blogging. */