Rendering is the step that turns code into the page you see: a browser receives HTML and then runs the CSS and JavaScript. Search engines render pages too, but not every bot does. According to its JavaScript SEO guide, Google processes JavaScript in three phases: crawling, rendering and indexing.
Pages with status code 200 go into a rendering queue. That can take a few seconds, but also longer. A headless Chromium then renders the page and Google uses the rendered HTML for indexing.
Why does it matter?
If your content only appears after JavaScript runs, it depends on whether a bot performs that step. Google itself writes that not all bots can run JavaScript, and therefore still calls server-side rendering or pre-rendering a good idea.
Example
This is the HTML a bot receives for a page that is built entirely in the browser:
<body>
<div id="app"></div>
<script src="/app.js"></script>
</body>Without rendering, there is not a single product name, price or explanation here.
How do you check it?
Compare the page source (in your browser via “view page source”) with what you see on screen. If the important text is not in the page source, it depends on JavaScript. In Google Search Console, URL Inspection shows the HTML Google saw after rendering.
More in Can AI crawlers read your website? JavaScript and server HTML.