SSR Server-Side Rendering
When the page is rendered to HTML on the server and sent ready to the browser.
Traditional web pattern (PHP, Rails, Django) that became fashionable again after the CSR wave. Good for SEO (Google sees the content immediately), fast first load (no JS needs to run before anything shows up), accessibility.
Modern frameworks (Next.js, SvelteKit, Nuxt) do hybrid: SSR for the first load, then CSR for subsequent navigation.