The same Origin policy is a feature adopted for security purposes that allows a web browser to run scripts from one webpage to access the contents of another webpage provided both pages have the same origin. The URL scheme, hostname, and port number combo are referred to as the origin. This policy was introduced to prevent access to sensitive data on one webpage by another for ill purposes. Consider a Java program used by scaler.com, the program can access domain pages like scaler.com/mentors, scaler.com/courses but none from different domains like facebook.com.
The Selenium Server (Selenium RC) acts as a client-configured HTTP proxy and "tricks" the browser into believing that Selenium Core and the web application being tested come from the same origin.
