Isomorphic JavaScript is the idea of writing JavaScript that can run on the server just as well as in the browser, thus your otherwise frontend-only single page app (SPA), which only makes HTML appear after some post-load requests and execution, can also render the same HTML server-side (which is how things used to work) and give something to search engines to understand. AngularJS required hacks using PhantomJS to achieve this, by running a headless browser on your server.
In theory, a full stack web developer then only needs to know one language - JavaScript. In practice, knowing...