Rendering is the process of executing a page's JavaScript to produce the final HTML content, which search engines must do before they can read JS-dependent content.
Google can render JavaScript, but it happens as a separate, delayed second wave after initial crawling — content that only appears after JS execution can take substantially longer to get indexed than content present in the initial HTML, and rendering can fail entirely for complex or resource-heavy scripts. This is the core reason server-side rendering and static generation remain preferred for SEO-critical pages over pure client-side rendering.
The practical test: view a page's source via "fetch as Google" or the URL Inspection tool's rendered HTML, not just the browser's rendered DOM, if critical content (titles, body copy, links) is missing from what the crawler actually processed, that content may be invisible to indexing regardless of how it looks in a normal browser.