Login attempts from non memers of my site


I have installed a plugin called 'Simple Multisite Login Log' log to see the names of users who has logged into my site.

The problem I have is, I'm seeing these usernames in the login log that are not in my users directory. They appear on the login log each day. Anyway it seems non of them were able to log in to my site as of now :slight_smile:

Who are these internet zombies and what harm can they cause to my site if they somehow manage to login?
Possible remedies to prevent these login attempts?

Thanks and have a nice day :slight_smile: