Websites used to be static html files.
replyYou either write them by hand, or use a tool that generates it locally, upload everything and you're done. Perfect security. Great performances.
It's in this sense that static generators go back to the source, the simply produce dumb HTML files that you upload/publish to a web server that doesn't need to run any code. Just serve files.
Imho CMS is just a tool that generates static html files on the server. The distinction is a bit artificial. CMSes have static html cashing and CDNs will allow you to "one-click" firewall the dynamic administration and cache the static html for you.
replyStatic website generators are cool way for programmers to do that work on their machine but in the end the distinction of what gets served is very small (if you set up the basics).