The preview is a live demonstration to the extent that it runs a real copy of the plugin, will can a real website, generate a real static copy in a real GIT account, and publish the static copy under a real Pages account that can be viewed independently on the Internet.

The only difference between the demo and the real thing is because the copy of WordPress loads inside your browser, it is not directly visible to the Internet (or to the web crawlers), as a result (in this instance) the plugin is hard-coded to scan this site. i.e. the demo will scan https://makemestatic.com, make a static copy of it, then publish it under a new unique URL.

This gives a representative view of what it does, how it works, the sort of output it generates and how long it takes (typically between a minute and a minute and a half end-to-end). 

For a primer on how to use it (which in this instance is pretty much “click NEXT a few times, then click Sync”, please take a look at the Getting Started page which provides a walk through with screenshots.