An email box monitor is simply an imap/pop3 account, just like a bounce or feedback loop server, where MailBul will connect from time to time and download the contents and process them in order to take a certain action.
You can define conditions so that while MailBul processes the emails found in the account, it will take certain actions if it finds certain content. For example, you can tell it to unsubscribe subscribers from the email list in case it finds the phrase “unsubscribe me”.
This feature is really useful when people reply to your email campaigns and tell you to unsubscribe them for example. This way you will spare valuable time since the process can be automated.