Server-side rendering (SSR) means the server builds the complete HTML of a page and sends it to the browser. The opposite is client-side rendering: the server then mainly sends JavaScript, and the browser builds the page itself. With SSR, the text is already in the HTML a crawler fetches. The web.dev article Rendering on the web compares the approaches.
Why does it matter?
Google processes JavaScript in three phases: crawling, rendering and indexing. Rendering happens once Google has the resources, so not always straight away. Google still calls server-side rendering or pre-rendering a good idea, because it makes your site faster for users and crawlers, and because not every bot can run JavaScript.
Example
Say an online shop only loads the product name, price and description through JavaScript after the page has opened. The HTML the server sends then contains only an empty container. With SSR, the same information is in the HTML from the start.
Common mistake
Assuming every crawler sees what you see in your browser. Your browser runs JavaScript; a crawler does not always do so. Which AI crawlers do and do not is covered in Can AI crawlers read your website? JavaScript and server HTML
How do you check it?
View the page source (not the element inspector) and search for a sentence from your main text. If it is there, the server delivers it. Atlas, the research engine developed by Outseen, investigates the HTML as the server sends it.