Upgrade key protects your Moodle site during the upgrade.
When the Moodle site is being upgraded as a result of the Moodle core update and/or a plugin installation/update, no authentication and authorization mechanisms are reliable. Any anonymous visitor of your site can potentially trigger the upgrade process by navigating their browser to your admin page. This can be seen as a security risk because a lot of sensitive information (server environment, plugin versions etc) are available at the upgrade screens.
To improve the protection of your site during the upgrade, it is recommended to configure the upgrade key in your main config.php file:
$CFG->upgradekey = 'put_some_shared_secret_here';
If the upgrade key is defined in the config.php file, its value must be provided to access the admin pages during the site upgrade.
Do not use your admin user password as the upgrade key.