It just means Rogean hasn't updated the security certificate, and it expired. You can ignore the browser warnings, and as long as you don't give the wiki your credit card (
[You must be logged in to view images. Log in or Register.]) you'll be perfectly fine.
(Security certificates are how websites encrypt sensitive information like passwords.
Hypothetically, if you login to the wiki while it's like this, someone could ... again,
in theory .. launch a "man in the middle attack" and steal your password. The chances of that happening are very, very low, but if you're truly worried you can still use the wiki, but just don't login until Rogean fixes it.)