Sorry I know other people have responded already, but it means you don't have to write any client side javascript at all. You can just skip along with a server that returns html. Just throw the script tag in there, and you're done.

The magic of this is that it's pretty easy to make SPA-like webapps with no javascript or complex client side framework. You can write your server in python, rust, clojure, whatever. If you don't need a lot of state management, it's really simple and awesome.