Web Application Inspector gives you information about what technologies are used on a website. Just click on the little icon that comes up in the search bar of your browser to see all of the applications that the Inspector recognizes.
It will tell you if it is built with Wordpress, Drupal, or Joomla for example.
It will tell you also if the site is using IIS, Apache, Nginx, for example as well.
The code makes all of it analysis locally, it doesn't make external calls. You can confirm this with TCPView or other tools if you are interested.
This is open source code that has been modified from the original work available at https://github.com/nqbao/chromesniffer. This modification includes an HTML icon which should show up on all websites even if the Inspector doesn't recognize anything else.