That conditional logic first checkes if the constant DOING_AUTOSAVE is defined by using the PHP function defined('SOME_CONSTANT_NAME').
Then the logic checks to see if the state/value of the constant DOING_AUTOSAVE is true, that’s the second part of the conditional right after the && (and).
So if DOING_AUTOSAVE has been defined in the request lifecycle and the state/value of DOING_AUTOSAVE is true then we return which is a means of aborting the logic that is to follow beneath the conditional.
DOING_AUTOSAVE only gets defined and set as true in the wp-admin/includes/post.php file within the wp_autosave() function.
Of course it may be defined elsewhere customly…
- Learn more about
wp_auto_save()here: https://developer.wordpress.org/reference/functions/wp_autosave/ - Learn more about
defined()here: http://php.net/manual/en/function.defined.php