What is apache tomcat? Can anybody explain this error message?

There is a website that I really really really need to get into, desperately. But when I click on the link, an error message appears: Error connecting to Apache Tomcat instance. Please check that a Tomcat server is running at given location and port.. Does that mean I'm not qualified to enter the site because I don't have a "Tomcat server", whatever that is? Is this problem solvable? What is a tomcat server?