Sometimes you want your application to remember where you were, but the application wants to redirect you to somewhere else.
This is most annoying when you sign in. You probably want to stay on the page you were on instead of being redirected back to the main page.
Rockets adds a global variable called (active-page). You have to set it before you call the navigation bar so it knows which page to highlight. But it also passes this variable as a hidden field in the navigation bar itself, alongside the user email and password, so that the verification script (rockets-verify.lsp) can redirect you back to that page if you either succeeded or failed in signing in.