Two Stage authentication which is otherwise known as Dual Factor Authentication is where you try to login with your credentials and then the site sends you a code that needs to be entered as well. This code can normally be sent to you using an email or via SMS, some systems allow you to use a code generator application that runs on your phone (such as Microsoft Authenticator, Authy and similar). If you dont enter the correct code in you do not login.
I am not familiar how this forum is using DFA as I have not enabled it on my account. You can enable it by going to your Password and Security settings.