Server Side Rendering is a concept where the server gets more responsibilities, namely rendering the application on the server and sending the fully rendered page back to the client. By doing this the user will see a rendered page more quickly, before the bundles are loaded and the application has started client-side. Since the page that's returned by the server is fully rendered (the page content is already in the HTML), your site will receive a higher ranking by search engines.
Server Side Rendering is only feasible because of something called isomorphism. An isomorphic framework allows you to run one application both on the client and the server. Aurelia is such a framework.
