At its core, it establishes software components called "attesters" that decide whether your device and/or browser is "trustworthy" enough - as defined by the website you are trying to visit. Websites can enforce which "attesters" users must accept, simply by denying everybody access who refuses to bow down to this regime; or who uses attesters that are deemed "inappropriate"; or who is on a platform that does not provide any attesters the website finds "acceptable".
In short: it is specifically designed to destroy the open web by denying you the right to use whatever browser you want to use, on whatever operating system. It is next-level "DRM", introduced by affiliates of a company that already has monopolized the browser market. And the creators of this "proposal" absolutely know what they are attempting here.