This page is only enabled if your server enabled SSL and set a signup_port. It can be used to register accounts for your server. Feel free to change anything in the html folder, but don't delete any default files - or the website may break.
When you try to join the server, you'll have a chance to put in a username and password. You can join as a guest by not entering a password when prompted. Or, you can create an account here to play with an account.