Server-side rendering (SSR) is the process of generating HTML on the server, and is what SvelteKit does by default. It's important for performance and resilience, and is very beneficial for search engine optimization (SEO) — while some search engines can index content that is rendered in the browser with JavaScript, it happens less frequently and reliably.
That said, some components can't be rendered on the server, perhaps because they expect to be able to access browser globals like window immediately. If you can, you should change those components so that they can render on the server, but if you can't then you can disable SSR:
export const ssr = false;Setting
ssrtofalseinside your root+layout.server.jseffectively turns your entire app into a single-page app (SPA).