The difference is more heuristical than technical.
Render the entire page in one go, rather than have the server render small chunks at a time and have the client patch the DOM. I.e. not having to think about below the fold, adding event handlers to pre-empt and preload. In practice, you’ll need to sprinkle in JavaScript interactivity, but the point is keep this to a bare minimum. See Craigslist or HackerNews.
I thought if you're using server side rendering you'd be serving HTML directly from your application.