The idea behind progressive enhancement is that you start with a web page which doesn’t require any JavaScript, and for that matter, CSS, and you enhance the page depending on what the browser allows on the user’s device. This allows older devices to continue to use the web and saves on people’s resources as they no longer need to update their devices in order to keep up with the modern web.
Progressive enhancement typically has a focus more on the content of the page rather than the bling of the internet.
A great book on the topic is Adaptive Web Design: Crafting Rich Experiences with Progressive Enhancement by Aaron Gustafson.