What the difference actually is
There are fundamentally three ways to render a page, and most everything else is a variant or combination of them. With client-side rendering (CSR) the server sends an almost empty page and lets the browser build the content with JavaScript afterwards. With server-side rendering (SSR) the server sends a finished HTML page per request, with the content already in the response. With static generation (SSG) the HTML is built ahead of time at publish and served the same to everyone. For the visitor the difference shows mainly as speed: a blank page that fills in afterwards feels slower than one that arrives ready. For a search robot that same difference decides whether the content can be read at all.
Why it matters for search
A crawler wants the content in the first response. If it can read the text, the headings and the links directly in the HTML, the page is indexed quickly and predictably. If the page instead builds its content with JavaScript after load, indexing becomes delayed and unreliable: the robot has to re-render the page in an extra step, and that step is neither guaranteed nor fast. A site that is fast, crawlable and clearly structured was an advantage in classic search and is a requirement now that answers are increasingly written by a model that only cites what it can read directly.
A page built in the browser doesn't exist for the robot until it has been re-rendered. A page built on the server exists straight away.
When SSG is enough, and when SSR wins
This is not a matter of faith. Static content like a knowledge article or a service description does beautifully with SSG: built once, fast everywhere, trivial to cache. Content that changes per visitor or has to be fresh to the moment belongs in SSR. In between sit hybrids like incremental static regeneration (static that refreshes in the background), streaming and server components, but they all rest on the same three base modes. Most marketing sites land on a mix, and the point is to choose deliberately per page type instead of letting a framework choose for you. Web development is decisions about trade-offs, not tech religion.
How we think about the choice
When we build a site that has to rank we start from a simple rule: the content should be in the HTML in the first response, whether it comes from SSG or SSR. Then Google can read every page directly, index it and start showing it. The rendering choice is therefore not the final polish on a build but one of the first SEO decisions you make, and it shows immediately in how quickly a new site goes from invisible to indexed.
If you want to build a site where every page appears in search from the first response, the web work starts in exactly that decision.
