A little history
Back in the days, rendering a website was simple. You needed a web server that served HTML files. Those were static sites. Then developers started using databases and authentication. To achieve that, they needed to manipulate the HTML file before serving it. That's how server-side rendering was born. Let's fast forward until 2010, when Backbone got released. The front-end got richer and more complex. Then the era of client-side applications begin. Developers migrated their data and routing logic to the client side. They could, because Google "understood" JavaScript. The servers became slimmer, but the websites became more complex. Yet, recently server-side rendering became a trend again. All thanks to React and its server-side hydration feature.
Static sites
Static sites are the simplest way to render a website. You code your website in HTML/CSS, and serve those files from a web server. This is the simplest way to render your website, but it comes with pros and cons.