The CLI Authorization Key is used to run the cron script over HTTP. Normally the cron script will run only if you are logged in or if you are using PHP command line. This will be a problem if you try to run it over the web without logging in, so use the CLI authorization key as a GET variable (key=xxx) in the web script URL so that the script will execute without logging in.
A new key will be generated if you use the 'Reset CLI Key'. Please remember that resetting the CLI key could break your cron jobs if you don't provide the new key to your calling script.
It is recommended that you avoid using CLI key and use a PHP command line script to run cron.