Right now, you go to a url, which has a server behind it. Your browser says, "I'd like the site!". The server gives you the site, and your browser shows you the site and runs it for you. When you run the site, you may actually make modifications to the site in your browser, blocking ads, doing some custom stuff with extensions, etc.
Now, Google is essentially bringing something into their browser that allows servers to block any modifications to the site as you run it. And servers may automatically block any browsers that don't support that feature. Basically, servers will be able to ensure that you run the app in the exact way they intend, without modification
Oh, and why is bad: the server requires your browser to pass a check to see if it's going to run your code correctly (called an attestation). It asks a third party to do that check, which will most likely be Google. Guess which browsers are going to pass those attestations more easily? Chrome. The only possible benefit to the average person is that maybe this will make it more difficult for bot farms to access the web