It means that while the web server was trying to generate the content for the page you tried to visit, some sort of serious error happened. That usually means a bug in the software that generates the web page, but it could be some other server related issue (e.g. out of memory or disk space).

If this isn't a web server you own or have access to, there is nothing you can do about it except maybe contact the site owner and report the problem. They may already know about it and be fixing it. Try again later.

If you are the developer, it means you have a problem. If you view the same page from a browser on the machine hosting the site, by using http://localhost then you will see the error details on the screen, rather than the generic page. You can also enable showing the errors to everyone by making the web.config changes, but that is not generally a good idea. Error pages give hackers hints about your system they don't need to know.