Progressive enhancement is a methodology for developing web applications. Using this method, you will be able to serve your website to anyone on accessibility spectrum.
The main objective using this methodology is to implement the core functionality first. This means that without additional styling or the use of programming, the core function would still work. So, in short, core functionalities have function without additional technologies and, when applicable, additional technologies such as styling or programming enchance the core funcionality.
Due to the increasing variation in browser(technology) and userbase, it is important to understand what the core functionality of the web page(s) are. Understanding the purpose, and so the core functionality, allows us to develop the page for any type of user (on the accessibility spectrum).
It starts with (semantically correct) HTML that functions properly on any browser. This results in enabling all users to achieve or use the core functionality despite browser or used technologies. It just works.
The second layer, the usable layer, focusses on the usability of a page. With proper styling, the page becomes more accessible to users with, for example, visual impairments. It is, however, not limited to just styling. Modern CSS techniques allows for optional, accessibility specific features.
The third and last layer is the pleasurable layer, this layer focusses on the pleasurable part of the website such as animations. This layer consists of additions to the functional features from the first layer.
Metaphor explenation link: Hotdog