I'm 48 and I get this (although I prefer Hugo and Github pages).

What part is confusing to you? I'm happy to help.

Basically you provide the app a folder of files and that app transforms the files into web pages and then (maybe?) serves them.

It let's you create a simple web site without writing any HTML. You just provide a folder of files (text, photos) and choose a template, and it'll turn that into a web site for you.

"Static Site Generator as a Service."

It's like if dropbox automatically exposed your files on the web.

I think you mean when, not if.

The public folder feature was really nice.

Drop had that features. I was warned multiple times when I started hosting my file download away from my website host to Dropbox.