if ( ! defined( 'WPINC' ) ) die; and
if ( ! defined( 'ABSPATH' ) ) exit; add an extra layer of security by preventing any direct access to your plugin file.
ABSPATH is a PHP constant defined by WordPress in its core.
If your plugin file is accessed from outside of WordPress, the constant
WPINC will not be defined, so it exits the plugin code, preventing any unauthorized access to your code.
WPINC are defined in WordPress core as:
define( 'ABSPATH', dirname(dirname(__FILE__)) . '/' );
define( 'WPINC', 'wp-includes' );
Both are used for same purpose.