Each time a user tries to log in on our wiki, the user is forced to use a secure connection. This way his password will be sent encrypted, otherwise it would be sent plaintext which is quite insecure.
An SSL connection requires a server side certificate which was signed by a certificate authority. Nearly all of these authorities require money to make them sign your server, but there's an alternative for smaller organizations: CAcert.
Browsers don't have CACerts stored in their stock database of ssl certificates, so each time you visit a CACert powered ssl site you will get a security error, complaining about an “untrusted certificate authority”. To get around this, the browser offers you to create an exception for that specific site (which stays permanent). You have to do that if you want to log in on our server.
However, it's perfectly safe to import the CAcert Certificate into your browser, so you won't need to create exceptions