Doesn't mean anything. It's usually "web server 2" and noticed on websites implementing load balancing, usually high traffic sites like mid-day.com
3, 4, and so on usually denote the web server being used to deliver the page. Each web server is a sub domain, "www2" etc. Very common on sites using load balancing. See http://en.wikipedia.org/wiki/Load_balancing_(computing)#For_web_sites