This is the Sign In page (Signin.tml/Signin.java).
You were sent here because you requested a page that requires you to authenticate (log in). In this app, all pages require you to log in except those whose Java class is annotated with @AnonymousAccess. We've created a "component request filter" (AuthenticationFilter.java, in this case) that enforces that rule. It redirected you to this page because it did not find the the @AnonymousAccess annotation in the Java class of the page you requested.
See here for more information on this and other security approaches.
One final note: the name of the page you requested was appended to the URL of this page, and it will also be passed along when you submit this form, or when you click the Register link, so that eventually, after you have registered and/or logged in, you'll be taken back to the page you wanted to go to.