The line effectively serves as a security measure. It ensures that the script can only be executed if the ABSPATH constant is defined. If someone tries to access the script directly (for example, through a web browser), and ABSPATH is not defined, the script will terminate immediately, preventing unauthorized access or execution of the code.

In summary, this line of code is a safeguard to ensure that the script is being run in the context of a WordPress environment, and it prevents direct access to the script, which could lead to security vulnerabilities.

defined('ABSPATH'): This part checks if the constant ABSPATH is defined. In WordPress, ABSPATH is a constant that represents the absolute path to the WordPress directory. It is typically defined in the main WordPress configuration file (wp-config.php).