Single sign-on (SSO) is an authentication scheme that allows users to login with a single ID and password to more than one application. You login to su.domains with your Stanford SunetID, which is the same credential you used to login axess.stanford.edu.
Stanford Domains is a service for Low Risk data. There is no need to protect your website contents. However, if you want ALL of your website content available to only Stanford Community, you can setup Single SingOn for your website. For Medium Risk data, please use Stanford Sites service