Debug settings

> Enable the “debug” mode throughout WordPress (WP_DEBUG, false by default)

(When this setting is enabled, WordPress displays all PHP errors, notices, and warnings.)

> Save all errors in a log file (WP_DEBUG_LOG, false by default)

(Is a companion to WP_DEBUG that causes all errors to also be saved to a debug.log log file This is useful if you want to review all notices later or need to view notices generated off-screen.)

> Enable display of error messages on your site. (WP_DEBUG_DISPLAY, true by default)

(Another companion to WP_DEBUG that controls whether debug messages are shown inside the HTML of pages or not.)
(If used on a live website this could lead to the disclosure of sensitive information about the website and server setup, so use cautiously.)

> Enabling this feature, WordPress uses the developed versions of CSS and JavaScript instead of compressed versions. (SCRIPT_DEBUG, false by default)

(This is useful when you are testing modifications to any built-in .js or .css files.)

> Enable a database query log (SAVEQUERIES, false by default)

(This option comes in handy if you are experiencing WordPress database issues.)

(NOTE: This will have a performance impact on your site, so make sure to turn this off when you aren't debugging.)

For more information wordpress.org