Backdoor is the Joomla 4/5 kompatible Version of the famous AdminExile plugin from Michael Richey (RicheyWeb).
Backdoor protects your backend / administration part of your site by using URL access keys (query parameters).
So, attempts to access the administrator login page will be met with either a redirect to your homepage, a 404 error, or a redirect somewhere else. This is e.g. a way to prevent brute force attacs.