Static sites are the simplest and most secure websites. Static means there isn't any programming language executing on the server to render the page. The web server only has to read the requested HTML file and return it.
Sounds boring, but complex sites can be static by using a static site builder, like 11ty, and adding functionality through external 3rd party services to handle stuff like forms and search.
Static sites are a great option for content heavy websites or websites that don't change often. I think a lot more website should be static.