This is a simple module that will stop users from being able to login as user 1. Administrators will still be able to login as user 1 using drush. The idea behind this is to protect a site from the accidental loss of user 1's password. Ideally, two-factor authentication would be used to mitigate that instead, but this is a quick alternative.
If you enable this module, and do not have access to the machine using drush— you will NOT, I repeat, NOT be able to log back in as user 1. This will be sad for you, but here is your warning.