Normally, when a non-authenticated user navigates to eXo Platform, he will be redirected to the Login form of eXo Platform for authentication. In eXo Platform, the CAS add-on enables the seamless integration between eXo Platform and CAS SSO Framework. So with the enabled CAS integration, the user will be redirected to the CAS login screen where he provides his credentials rather than eXo Platform login form. By logging into CAS, he will then be redirected to eXo Platform without login again. See here for details about CAS.
The integration between eXo Platform and CAS consists of 3 steps:
i. CAS is deployed on Tomcat 7 server at localhost:8888.
ii. eXo Platform (Tomcat or JBoss EAP) is deployed at localhost:8080.