This file is automatically added, and generally also removed, when you use a plugin for maintenance mode. WordPress will also load it if it was manually generated. The received timestamp should be released, but you shouldn’t forget to delete the file after the maintenance is completed.
Another option for manually starting the maintenance mode is conducted with the functions.php file. functions.php is a template file that is used to add functions and features to the WordPress project. In this case, you enter a PHP code into the file, which serves to activate the maintenance mode. There are different types of such PHP codes. A possible entry into functions.php looks like this, for example: