A static website is a website where the initial HTML is prepared ahead of time, not dynamically generated by a server on request.
Better yet, static sites let you shift computation (where possible) from the user to the server. Sites like this one where all pages are rendered on build. Plus, it gives you additional resilience to mistakes; a production error that crashes a build doesn't impact a live site, and even live errors can simply be rolled back one deploy if you're using a service like Netlify.
Static sites are more performant, more resilient, have better SEO, better user experience, better accessibility, they can be more easily cached. Yes, there will always be some web uses which don't fit the model (a static Slack would be a nightmare) but for a lot of websites, it could be a better choice, particularly as we get features like Incremental Builds.